A big part of that is that Apple controls the horizontal and vertical (sorry if you are too young to get the reference). I would imagine they have customized store builds that they can easily restore to a normal state when someone royally screws it up. Then there's probably a level of reasonable user level protection setup (no admin access). I've never tried going into an Apple store and tried to do admin type things. I'd be interesting to see how they deal with that.

The contrast is even sharper in other countries. In some places it is a huge cultural no-no to go into a store and start touching stuff. Just try going into a toy store in one of these places with a US-raised kid used to ToysRUs.