NTR is in the uncanny valley of giving a fuck.
There's content that drives 90% of the site's viewers away in disgust and horror (myself included). And there's tags that people ignore because they're common, or because they're not THAT bothered by it. At its most basic, oversimplified to the point of incredulity, NTR is just sex between "the wrong people."
There are people who are really into it, people who are super against it, people who don't mind it in certain contexts, and people who literally could not care less. Of those four groups (and I'm sure there are more) none of them recoil in disgust when they see the tag--on the contrary, the first two are compelled to start posting. Even the people who are just mildly against it might join the thread, complain, ask for it to be made optional, and then shut up about it if they CAN avoid the content.
So it's not offensive enough to drive people away, but it IS offensive enough to piss a lot of them off.
It's the perfect middle ground for rustling jimmies without actively trying to agitate the audience through the use of cliffhangers, red herrings, or baiting.