How can they even get away with that? If they tried pulling this shit with Kermit the Frog or Bugs Bunny they would get a cease and desist injunction, wouldn't they?
Several reasons.
One, who are you gonna serve that C&D too? These are many, many anonymous online posters, remember, doing this
en masse. I guess you could serve it to hiro (4chan's current owner) or serve a DMCA take-down request, but those take so much time, that I'd guess you'd never get whoever is behind 4chan staff-wise to capitulate to constantly patrol for use of your content. It also goes against 4chan's stated
laissez-faire attitude towards posting and content, so they're must less likely to heed any C&D warning, imo. Your C&D idea might have more weight with sites with more regulating moderation and staff, like NeoGAF, but I have no idea.
Two, fair use. They're not making money (in theory) off them and are remixing them. They could probably argue fair use in a hypothetical court case.
Three, the original artist is really small time. He has neither the time nor resources to hypothetically patrol this, especially if the Henson Co./Warner Bros. really don't go after memeic remixes either.