So how would you define 'obscenity', then? Are you sure that others would define it the same way? After all, what you consider obscene may not be the same as something I consider obscene.
This kind of thing plays into the controversy between the rights of a business to be selective about its clientele and the rights of the individual to equal treatment under the law. Can a private business restrict the kinds of clothing worn by its customers? Even if the customer feels he has the right to wear what he wants? Many businesses will not permit people to enter without shirts, or shoes. Are they guilty of discrimination?
Personally, while I deplore the kind of mentality that would want to cover up such a shirt, I see no problem with a privately owned business having a dress code for its customers. If you don't like it, don't go there.