That's half the fun. Watching them squirm and try to figure out how to explain the joke without saying the -ism out loud when they realize they're not in "safe" company.
I have a lot of jokes where my friends don't find them funny. But there's always a punch line, some smart wordplay, terrible pun, even if they feel like it's too forced (smh).
Racists do know that openly saying that "minorities being abused is funny" sounds racist as fuck, so they try to avoid explaining their jokes.