Which is funny, because the NSFW rules state that it's not only porn, but profanity as well:

NSFW (Not Safe For Work) content
Content that contains nudity, pornography, or profanity, which a reasonable viewer may not want to be seen accessing in a public or formal setting such as in a workplace should be tagged as NSFW. This tag can be applied to individual pieces of content or to entire communities.

Literally according to Reddits own content policy rules, any sub that allows swearing should mark itself NSFW - which basically means all of them because I've yet to find a sub where swearing wasn't allowed.