Is sleeping on the floor in planes allowed, and if not, why not?

There is no clear explicit rule regarding sleeping next to bulkheads. Why are some airlines strict about it? the usual "safety" excuse is used here and I can assure you this time the safety excuse is legitimate. In addition to that, passengers are only allowed to sit in "passenger seats", they are not even allowed to sit on crew seats, so this can be used as a reason to prohibit people from sitting/sleeping in places that are not certified for passenger use, from FAR part 121.311:

• (a) No person may operate an airplane unless there are available during the takeoff, en route flight, and landing --
  • (1) An approved seat or berth for each person on board the airplane who has reached his second birthday; and
  • (2) An approved safety belt for separate use by each person on board the airplane who has reached his second birthday...

In addition to the above, there are rules that prohibit storing luggage next to bulkheads, because things there will fly in case of severe turbulence due to the space. The same thing can happen to sleeping beauties there. If this were to happen it will not only harm the sleeping person, but also other passengers. Also, when you are laying on the floor there you are next to tons of metal objects (seat legs and so) they are not really sharp, but I can imagine a simple contact with these metal objects either by turbulence or by mistake will lead to injury.

Anyway, in the airlines I work for, this is tolerated. I guess why sleeping there is tolerated in some airlines and not in others is the culture of suing. In the US if something happened to a passenger he/she will surely sue the airline, while in other cultures things are not like that, hence why airlines tolerate or not regarding this and other things as well.

I've had it specifically announced on a flight was that it was for safety reasons, including:

1) Blocking access to the bathrooms
2) Preventing drink carts from moving up and down
3) Safety risk in the event of turbulence

This was definitely on a flight in Asia, but I'm sure I've heard it on a Qantas one as well.

I've done it, next to an emergency exit even. But my seat was such that half of it was blocked by the emergency slide, and after asking the cabin crew they said it was ok for me to sit on the floor instead except when the seatbelts sign was on. That seat was later removed from the seating plan btw, guess I wasn't the first or last to complain about it. In any other situation, I've never seen the need to not use a seat, except for a few minutes of stretching.

Safety is a concern, as said already. Obstructing the passage of other passengers is also a concern. If you're sitting on the floor, passengers need to climb over you to get to or through the aisle. And if in the aisle, you're hindering cabin crew trying to serve drinks and stuff too.

On most of the overnight flights I've been on, the cabin crew have announced that passengers who want to sleep should have their seatbelts on, and visible over their blankets, etc. It's also recommended that you keep your seatbelt on, in case of sudden and unexpected turbulence. In turbulence, passengers need to be seat-belted to prevent them bouncing around and injuring other passengers and the crew, let alone themselves, so the crew check that everyone's wearing one when the seatbelt sign is turned on.

A passenger who's sleeping on the floor, even if not in the aisle, isn't wearing a seat belt. They're also fairly unlikely to hear the gentle ping of the seatbelt sign coming on, so will remain unrestrained until the cabin crew wake them and they slowly and groggily return to their seats. That means the cabin crew are at greater risk of injury because they have to spend more time walking the cabin checking passengers while the seatbelt sign is on. And, if turbulence happens suddenly, an unrestrained passenger in a relatively open space is likely to move farther than one in the pretty confined space of their seat.