What's so bad?

When Youre All Dressed Up, You Need Somewhere to Go
There is nothing wrong (or illegal) for a restaurant to require a certain level of sartorial decency in order to be served. I once went to a supposedly upscale place for dinner only to be seated next to a couple, one of whom was in shorts. On the way out, I said something to the manager; and I can promise you that I will not go there again.
I am no prude, and I do not go out of my way to wear a jacket an/or tie to go out. There is nothing wrong with casual dining. But it shows a certain level of adolescent self-involvement when you don't have enough consideration for others in the establishment not to look like a damn slob. It's the same with screaming children and rowdy (adult) behavior.