You're conflating what you can do and what you should do. For the vast majority of rights we have, I think we should exercise them on a regular basis, lest people think we "don't need them" anymore. But it's not our role to play criminal justice system.
Yes, you can discriminate against people by their race or height what they want written on their cake or because they looked at you funny. That doesn't mean you should
And yes, you can discriminate against someone who does those very things. That still doesn't mean you should.
(There's some asterisks here, of course. You always must fulfill agreements, offers, and other contracts. The law might disagree with me, but that doesn't make the law right.)