The scene last weekend: three cramped people trying to sleep in three bulkhead seats with those annoying immovable armrests and a whole lotta legroom in front. Why not curl up for a nap in all that wasted space?
Now some airlines (eg. Qantas) are quite strict about not allowing sleeping on the floor, while most quietly tolerate it, especially for kids and when the lights are turned down. But what's the rationale behind the ban, and is it universal? "Safety" was all I could get out the JAL flight attendant last weekend, but a quick Google search found only lots of idle speculation, not anything concrete like eg. FAA regulations.
Obviously there are no seatbelts etc on the floor, so it's not a great position to be in if there's turbulence. But you're allowed to walk around the plane after all, and I'm not aware of any airline that actually enforces wearing seatbelts when seated and the seatbelt sign is off.
Edit for clarity: Obviously you need to get up off the floor if the seatbelt sign goes off, during takeoff/landing, etc. But I'm still waiting for a non-speculative justification of why you can't lie on the floor when the plane is cruising along smoothly and passengers are allowed to wander about the rest of the plane.