The same thing with Social Justice Warriors. It makes it sound like they're heroically standing against injustice.
That's the core of the insult - that's what the people think they're doing, when what they're really doing is fellating their own ego.
That's right. The terms "social justice activist" or "social justice campaigner" are never used as insults. And that gets to the heart of what the term SJW as a pejorative means and why it's different from those two terms. It means someone who's jumped on a bandwagon and is self-righteous about it.
It's the same type of insult as "bible basher" to describe overly zealous Christians (new converts?) who constantly try and convert people and have a big ego because they're the true believers and everyone else has those inferior everyday beliefs. It's the innate sense that they think they're better than everyone else because they jumped on whatever specific bandwagon they happen to have decide is the "one true belief system". Bible Bashers and Social Justice Warriors are actually virtually identical. Both are the
same in terms of psychology. If you switch out the right nouns/phrases you could convert utterances between them seamlessly. It's just human psychology: groups and belief systems, they are all prone the same human foibles. There's nothing "special" about religion. Thus, other types belief systems can also fall prone to the same foibles that beset religious belief.
You can find the exact same flavor of asshole in virtually any context. Their are self-righteous assholes about D&D stuff, there are self-righteous assholes about Linux stuff, there are self-righteous assholes about Christianity. SJWs just happen to be the exact same type of asshole, but they've chosen that thing to obsess over. If you twitter-mob someone for saying the wrong words then that's not "brave", sorry SJWs. That's hiding behind mob bullying.