Banning NSFW content has nothing to do with "reddit users being too cheap". It also doesn't cost reddit anything since content moderation is mostly done by moderators for free.
That one is specifically annoying because "NSFW" is such a generic filter that doesn't always have something to do with sexual content. Some communities mark themselves as 18+ simply as a safeguard against SOME media they talk about that can be rated 16/17+ (R/M/MA/etc). But That isn't the content adverts are worried abobut.
I think there is some perceived advertisement value. And some sales person might tell management that just banning NSFW might help pushing some number of more adds... Thus to get numbers up ban it.