Know Your Meme's community is small. In terms of people "who are from the Internet," there aren't many even in the article comments, which tends to be separate from the forum community.
But for trending and notable entries (and many lesser known ones,) the admins and mods do a great job in doing research, making an easily read and well-enough edited article, and following a tried-and-true format in documenting the web phenomena.
Also, remember the full nature of KYM. Over a half million people/accounts "Like" Know Your Meme on Facebook. So even if they don't come by the site itself, they get KYM information in their newsfeed. So they have come to expect hard-hitting meme images such as the most recently posted one:
They likely aren't going to be terribly surprised by some off-color jokes. They would have unliked the site long ago if they were.
And the BBC has interns and reporters who know "the Internet." They know that many people who post in comments sections anywhere are going to be dumb. And the forums are still pretty hidden to most. We are far from important to the site. If you follow the habits and actions of the staff, most of the focus in terms of work and effort is near the articles. The forums are almost strictly an on-site KYM community thing. There often isn't a link to the forums from articles, and most people who visit will avoid Riff-Raff anyway, because it just plain won't make sense with all of the in-jokes. No one's going to make an account if they didn't have one to insult or cringe at you. Most people don't care. At worst, they leave the geeky kids alone.
And most of all, as others say, you're not going to find many relatively unbiased information on memes anywhere else. KYM serves and fulfills a very particular niche. It's not a stretch to say it serves a role that no other site can at this point: consistent, well-written, and easily (relatively) searchable articles on Internet phenomena.
