Why should the person denied service be expected to respect that service’s choice?
Because life is too short; there are likely other choices; and being respectful in general toward others should be the ideal.
One should be indignant, sure, but throwing a hissy fit just makes them look like a child and accomplishes nothing. And you don’t want to give someone incentive to pee in the soup you asked them to make (a la Fight Club).
JEDIT: I’m not saying they should put up with it. I’m saying standing there in the store having a shouting match about it isn’t going to get the customer what they want.