Do we really want to be in a place where this is necessary?
Assuming that it is necessary. Much of this is probably driven by lawsuit avoidance. It's tough to do business in America these days, sad to say.
Why do businesses that serve the public, have the right to discriminate against legal Firearm Owners?
This is a leftist trope that has been adopted by a lot of people who should know better, through constant repetition from the Ministry of Propaganda. Of course, "businesses" are in fact, just people. And people may discriminate against (another word for "choose to associate with") any person they please, for any reason they please - even if a tyrannical government says otherwise. It's nobody else's business if they do. The fact they "serve the public" is completely irrelevant.