What about business establishments that have a sign saying, "We reserve the right to refuse service to ..."? Is that legal or illegal? Or does it just depend on the business?
This depends. For example, refusing to serve alcohol to an obviously drunk person is required by law. Refusing to serve a person with no shirt, and no shoes is considered status quo due to health concerns.
However, if you refuse to serve based upon race, sex, religion, sexual orientation, ancestry and a whole mess of other criteria; you are discriminating and will be held accountable.