I don't think this thread's issue is about the kid being barefoot, but about the potential safety danger of going without shoes, and the potential of stepping on something that would cause injury. A piece of broken glass, a sliver of metal, a nail, a caustic liquid - any of these which could be in the parking lot, or even the store floor. It's only common sense on the part of the parents to protect their kids from any kind of injury that could be caused this way.
If they want to let their kids go barefoot, then make sure they're small enough to ride in the store cart, from the time they get out of the car until they get back in. Too big for the cart?? - then make sure the feet are protected! It's only a matter of common sense and has nothing at all to do with being immaculately groomed.