They've literally thrown old people in jail because they worked with Nazis once upon a time. It's perfectly acceptable to "punch a Nazi." You could probably even kill one and nobody would care. How is it not libel or defamation to falsely label somebody as a Nazi? It's dancing on the line of inciting violence against them.
Same thing with calling somebody an incel. Incels are associated with domestic terrorism in the mainstream consciousness. To call somebody an incel with no justification is to place them alongside terrorists.
When somebody calls you a nazi or incel or whatever else, they're basically painting you as a target that should be destroyed by the system. I used to just laugh it off but that was before these faggots started ruining people's lives and destroying the country. Now it's basically an attempt at your life. Fuck me? No, fuck you.