Do they have to take checked suitcase off the airplane, if passenger does not show up at the gate? [duplicate]

This rule was instituted after the Lockerbie bombing. Pan Am flight 103 was a multileg flight from Frankfurt to Detroit, via London and New York. There was an aircraft change at London. A passenger, booked to travel from Frankfurt to Detroit, loaded a suitcase bomb onto the first aircraft at Frankfurt and he himself travelled with it as far as London. At London he deplaned and left the airport; but his luggage was automatically transferred to the next flight as he was booked through to Detroit. The bomb detonated over the Scottish town of Lockerbie, killing all aboard and eleven on the ground.

In response to your specific question, I surmise that the Ryanair agent did not want you to think they would remove the baggage, because this would cause further delay; therefore by pretending that you would be parted from your baggage, you were persuaded to stay on board. I can assure you that if you insisted on deplaning and left the airport, they would remove your baggage from the hold (or if they didn't they would be in a lot of trouble for it).

The need to unload the baggage of passengers who don't show up at the gate is driven by concerns about bombs in checked baggage. The general principle is:

It should be impossible for a passenger to deliberately cause a bag he checked in to be carried on a flight that he is not himself on.

If someone could check in a bag and then have a reasonable chance of getting the bag to fly without him simply by not coming to the gate, that would be too convenient a way for terrorists to get bombs into baggage holds. Some efforts are made to screen baggage at the airports, of course, but the screening process is not perfect.

It used to be assumed that a terrorist would not be willing to go down with the plane he bombs -- in this age of suicide attacks this is probably not as airtight an assumption as it was once thought, but presumably the requirement to fly together with your bags still provides some kind of deterrent.

(Late and lost bags are routinely flown without being accompanied by their owner, but that's different in that the passenger cannot really do anything to make his bag be late.)

As the other answers explained, it is correct that an airline cannot fly with the baggage of a passenger who has decided not to board the plane.

The airline's refusal here is because they're not prepared to delay all the other passengers on the flight by potentially unloading and reloading all the bags and that they're unwilling to rebook the passenger on a flight the following day. Of course, if a passenger point-blank refuses to get on the plane, the airline cannot force them to travel and their bags must be unloaded. However, unloading bags for passengers who decide not to travel as a matter of convenience is not a service the airline wishes to offer for the convenience of one passenger, set against the huge inconvenience to all other passengers.