Joe Barry Carroll, meet Rosa Parks

I'm pretty sure you could've come up with this on your own, but let's go through it anyway. If a bar doesn't allow minors, there's a sign at the door. If a store doesn't allow pets or firearms, there's a sign at the door. If a movie theater doesn't allow children, there's a sign at the door. Once you pass the door and are served, the business owner has relinquished his ability to single you out unless you are acting improperly. The problem is, after a particular patron says no once, you survey the other patrons to see if one of them is willing to move. When a disabled person gets on a bus, the bus driver will ask the people in the handicapped seating section if they are willing to move, then continue to the next possible seat until someone is willing. They don't just stand in front of the first person and continually ask them to move, unless they obviously don't require special seating and everyone else does.

You have a point (even though you're off base in the implementation). A business owner can sell what they want to whomever they want, within reason. What they can't do is change the rules once you come in the door. Once you're allowed to come in and be a customer, nobody's dollar is worth any more or less than yours, and that's how you should be treated. Having your restrictions on the door also allows people who fit within the allowed group to see the type of establishment they are entering, and to decide if that's the type of establishment that they are willing to support.