Because the reasoning shouldn't matter for why you refuse service to someone. The government shouldn't have the ability to regulate the individual choices a person makes with their business because it offends a group.
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Why do protected classes deserve special treatment? What if I really don't like Jehovah's Witnesses because they believe weird stuff and their refusal of medical treatment offends me. Can I deny them service or do I have to serve them because they aren't one of the Left's privileged little voting blocs?