"People have the right to discriminate on their own property, even racially."
Statist: "Why do you hate black people?"
I believe in the complete abolition of drug prohibition. That doesn't mean I'm giddy with delight over the prospect of people overdosing on heroin. —George Geankoplis
The general error is that defending the right to do something is conflated with wanting to do it (or wanting to see it done). E.g., "A person has the right to discriminate based on race" becomes (after some mental gymnastics):
However, defending a right is orthogonal to expressing a desire. Just because something may be done in a free society—that is, it does not violate the Non-Aggression Principle — does not mean most or even many voluntaryists want to see it done. Racism, for instance, is mere collectivism: putting people into a group and judging them by a characteristic not relevant to what is being judged.
Although discrimination (and other non-aggressive acts) may not morally be responded to with force, there are alternatives to discourage it, such as:
Someone recently "went the extra mile" on this fallacy. An article on legalizing child pornography mentions that filming the sexual assault of a child by observing it while wearing video glasses would make the observer guilty of child pornography (strict liability disregards intent, and is common in the US and Europe) was posted, and, unable to come up with a rational argument against it, accused two other participants in the discussion of proclivity to sexual molestation of children:
"I'm going to go out on a limb here, but I'm guessing that people keep a close eye on you guys when children are present." (Mitchel Lewis)
Also, I suppose, a teachable moment, although I won't say I'm not angry at the ignorant personal attack. Let's get this out in the open. Mitchel Lewis accused two individuals making reasoned and logical arguments of being sexual predators merely because he couldn't manage a reasoned response (although he tried a few other fallacies first).
However, this is a similar fallacy to that of accusing people of being racist because they say private owners should be allowed to discriminate — except it's one level worse: it's to say that saying defending observing a racist act in progress makes a person a racist.
The reverse of this is sometimes, but more rarely, seen; the pattern "if you want X, you must support forcing X on people". For example, "if you are philosophically opposed to the death penalty, you must want to violently eliminate it".