You CAN walk into a store and eat things and just leave, and not even get in trouble lol. I mean, it's crazy in fact as at many stores they have buffet like areas with chicken/wings etc just there for you to take. You are supposed to go pay for it before sitting down to eat, but anyone could just snag a bunch, sit at a table by the starbucks etc, and chow down.
I'm specifically thinking of safeways but those are not the only stores with them (and safeways are everywhere).
I guess in theory you could just walk around the store with a cart putting food in it while snacking on grapes in your basket, or eating some cheese, drinking as soda etc, and then just ditch the cart, and leave. You are not leaving with any goods, so no reason to stop you.
Im sure people eat stuff in walmarts or other cheap stores cause you can see wrappers or open packages etc.