I don't think that it's unreasonable to ask them to move down. If Seat A is a well-known bad seat, then having them move away from it would be more appropriate.

I don't think I'd do it.

Then again, if most of the seats were open when they sat down, they shouldn't have left a one-person gap to begin with. Right after we teach people which side of the sidewalk to walk on and which door to open, we need to teach them some basic tenets of dynamic storage allocation.