Is it fair to buy a return ticket I know I will never use just because it's cheaper?

Flying one-way with many of the major carriers is normally far more expensive than half of a return due to the way airlines price flights. That leaves you with three options :

• Find a major airline that doesn't bump prices for one-way (as much). SAS is one that springs to mind, and their one-way flights are normally cheaper than other major airlines. Also look for airlines that are doing "tag" routes, or routes that are an extension to their normal flights. For instance, LHR-LAX one-way is almost always cheapest on Air New Zealand

• Use frequent flyer points, if you have any. Most airlines allow one-way flights on FF miles, and they are frequently one of the best way to use your miles.

• Check out the discount carriers, who normally price flights the same regardless of whether it's one-way or return.

If you're finding that the return ticket is cheaper than the one-way (not as unusual as you might think!) then you do have the option of buying a return and simply not using the trip back, however keep in mind that the one-way ticket will most likely have better conditions than the return. eg, the one-way - being a more expensive fare - will possibly be refundable and/or allow free changes (date, routing, possibly even cities). If there's any chance you're likely to use these benefits then it may be worth paying for the one-way - especially, if it's only a little more expensive.

If you do decide to go ahead with a return, keep in mind that the return leg doesn't need to be the same as the outbound. You might be able to book ZRH-AUS on the outbound, and a cheaper/shorter flight such as JFK-FRA for the return which may give a cheaper overall flight. Given that you're not going to fly it anyway, it doesn't really matter where you're "flying"! Try a few options and see what you can come up with.

One major concern is that some airlines don't like it when you drop tickets (legs or entire flights) because it's cheaper for you. As a result, not turning up can be held against you and any frequent flyer program that you might hold with them.

I don't know of anyone personally that's had theirs cancelled or penalised, but have seen it mentioned frequently elsewhere online (like flyertalk or reddit's travel subreddit).

But then again, that's quite a saving. It might at least be worth looking into whether or not you can get a flexible return, so that you could change the date to perhaps use on the way back. Or at the least, cancel it with the airline to lessen any chance of them getting upset with you, rather than just not showing up.

Yes. You can and often should do that.

In some instances, you are actually legally required to have a return flight (for immigration) even if you plan to travel to a 3rd country. Of course, this depends on the country and your citizenship/visas &c.

If you have an A to B return ticket and you have now arrived at B and do not plan to use the return to A, call the airline and cancel. You will probably not get a refund on the ticket itself, but you can often get a refund on some of the airport fees. which is a nice piece of change for a 10 minute phone call.

Throwaway ticketing is against the rules, but it is rarely worth the airline's effort to pursue occasional violators. The sanctions, for the most part, are to withhold services or benefits. I've never heard of anyone being criminally charged in any country for skipping their last segment.

The airline has priced the round trip ticket below the one-way because it believes it can maximize its revenue this way, and when you purchase the ticket from them, you are agreeing to a contract with the airline that says you will obey the fare rules and the airline's Conditions of Carriage. For example, United Airlines explicitly forbids your scenario under Rule 6:

J. Prohibited Practices:

1. Fares apply for travel only between the points for which they are published. Tickets may not be purchased and used at fare(s) from an initial departure point on the Ticket which is before the Passenger’s actual point of origin of travel, or to a more distant point(s) than the Passenger’s actual destination being traveled even when the purchase and use of such Tickets would produce a lower fare. This practice is known as “Hidden Cities Ticketing” or “Point Beyond Ticketing” and is prohibited by UA.
2. The purchase and use of round-trip Tickets for the purpose of one-way travel only, known as “Throwaway Ticketing” is prohibited by UA.
3. The use of Flight Coupons from two or more different Tickets issued at round trip fares for the purpose of circumventing applicable tariff rules (such as advance purchase/minimum stay requirements) commonly referred to as “Back-to-Back Ticketing” is prohibited by UA.

With the usual caveat that I am not a lawyer and the following is not legal advice, the chance that you would be charged with a crime and an arrest warrant issued for you is essentially zero. Your contract with the airline is generally treated as a civil matter, not a criminal one. While the airline may have the right to pursue an individual customer for the occasional violation, it is unlikely they would make the effort.

People do get penalized for breaches, but the cases I have heard about are serial offenders— people who deliberately book multiple nested tickets or show a pattern of doing so over several years, or who are travel agents doing so for multiple customers, resulting in significant lost revenue to the airline. If you get flagged as a violator, the airline has several avenues of recourse; United spells these out under section K:

UA’s Remedies for Violation(s) of Rules- Where a Ticket is purchased and used in violation of these rules or any fare rule (including Hidden Cities Ticketing, Point Beyond Ticketing, Throwaway Ticketing, or Back-to-Back Ticketing), UA has the right in its sole discretion to take all actions permitted by law, including but not limited to, the following:

1. Invalidate the Ticket(s);
2. Cancel any remaining portion of the Passenger’s itinerary;
3. Confiscate any unused Flight Coupons;
4. Refuse to board the Passenger and to carry the Passenger’s baggage, unless the difference between the fare paid and the fare for transportation used is collected prior to boarding;
5. Assess the Passenger for the actual value of the Ticket which shall be the difference between the lowest fare applicable to the Passenger’s actual itinerary and the fare actually paid;
6. Delete miles in the Passenger’s frequent flyer account (UA’s MileagePlus Program), revoke the Passenger’s Elite status, if any, in the MileagePlus Program, terminate the Passenger’s participation in the MileagePlus Program, or take any other action permitted by the MileagePlus Program Rulesin UA’s “MileagePlus Rules;” and
7. Take legal action with respect to the Passenger.

Simple: Book a cheap return ticket, throw away the other half. You're not supposed to do this, of course, but unless you make a habit of it the airlines will not care.

Flights are cheapest when nobody wants to travel. Low season, midweek, departures and arrivals at inconvenient times, and awkward transfers all lower the price. Problem is, you're looking for a ticket for July (high season) with only one month's notice; it would've been better to book 3-6 months ago.

There is no issue and I personally often do this.

Try to book the flight back for the most plausible date of return. It may be eventually useful. Also note that even without specific flexible tickets, it is often possible to change your flight schedule any time for a little extra money.

Be careful about having both round trips originating from the same location. for example, your first ticket set may be round trip Hong Kong to Melboure. You discard the return ticket of that set (Melbourne to Hong Kong). If your second ticket set is also round trip Hong Kong to Melbourne, obviously you only want the return ticket.

However, not showing for the outgoing ticket (Hong Kong to Melbourne) while expecting to use the return ticket (Melboure to Hong Kong) may lead to an unpleasant surprise. The airline may cancel the entire second ticket set when it determines that you are a no-show on the outgoing flight.

You can try to buy a multi-destination ticket, it should be less expensive than a one-way and the money will be more useful.
But the company needs to fly to both destinations.
(Or try with a traveling agency.)