It has everything to do with the size. You aren't suppose to lay plasmas on their backs or screens for the same reason you don't want to move a huge piece of glass in the flat position. The smaller the piece of glass (assuming the same thickness), the less likely there will be any problem. The same goes for plasma TV's. Plasma TV's have two pieces of glass inside and the largest danger is that as the unit is being moved on its back/front, some torsion will take place and one of the pieces of glass will crack. Once the unit is in position (on its back/front), the chance that there will be a problem is much less.

p.s. Ever seen the lifts made by SVS and others that keep the plasma in the ceiling on its back and then lowers it into the room?
[Link: svslifts.com]