They explain:

Since Reddit Admins in their infinite wisdom, have told us not to mark the subreddit as NSFW under threat of ~~death~~ ~~de-snooment~~ suspension, I guess this shit is up to you guys to figure out if your post is NSFW or not. I mean.. It says in at least one place that profanity should be marked NSFW... But then the admins have been telling people it doesn't.. So who knows.. Admins, if you'd like us to mark the subreddit as NSFW we are more than happy to comply =)

Sadly, videos no longer have to feature John Oliver