Well, the short version of the answer to both is that when doing The Road Warrior they straight-up just got all of Lord Humungus's people's (y'know, the villains) outfits from a BDSM shop. And that film is seminal to the genre's aesthetics. So...lots of people in fetish-wear, and many of them villains.
The making them villains thing is also partially because that's just a general tendency in media in general. A lot of forms of media do, in fact, just use BDSM/fetish wear as shorthand for 'evil monster'. Post-apocalypse fiction does it more than some genres, but honestly superhero stuff does it almost as often and several other genres also dabble in it (a lot of neo-noir has some of this going on, for example). Now, speaking as someone into BDSM, that's kinda unfortunate and has some seriously unfortunate connotations in some ways. Though the ominous vibe is part of the point for some people, I suppose.
But more importantly, the tendency to make people in fetish-wear the villains is pretty much completely absent in Apocalypse World unless you bring it with you. Several playbooks have fetish-wear, bondage gear, and similar things as an option, and those are only as evil as you make them. Ditto illustrations, many of those using fetish-y stuff are those associated with the playbooks, and again only as evil as their player wishes them to be.