Because, of course, they know that abuse, including verbal abuse, is wrong. They've been told this since they were small children, if not by their parents, then by their teachers.
It's still the abuse, whatever the words are, that is the problem rather than the words themselves.
Saying fuck when you drop a hammer on your toe is not abuse. Calling someone a fuckwit in an abusive exchange is. It really is that simple as far as I'm concerned. You can be abusive without using "swear" words, and you can be polite using them. Context matters. Always.