I think they have a pretty strong anti-censorship stance, and due to the way the data is en/decrypted on the client side, they have no visibility what the data consists of.
On top of that, the data here (which is posted with the mega.co.nz decryption keys standard as part of the URL) has an additional layer of encryption by the uploader, so for all anyone can prove it's pictures of cats, and not anything illegal.
On the other hand, they know exactly what the uploader claims the files are. If they don't take the files down and these turn out to be real, they could be held responsible.
