Why are you giving the employees who follow rules a hard time?
Because sometimes the rules are pretty arbitrary and other employees seem to recognize that. I've worked retail and food industry, when that was the case I'd bend the rules if it was within my power to do so. Never got in trouble for it and I'd rather do that than have unhappy customers leaving without buying stuff or tipping me. Generally managers don't care if you have some comps if you're sales are better than everyone else's.