After the initial kneejerk "oh, the MPAA hammer has gone crazy again!", you should take this into consideration:
Youtube is host to some of the most vocal asshats and dipshits the tubes of the internets has ever seen. I'm sure that the person that posted it didn't disallow comments, so it's only a matter of time before you see comments to a Disney cartoon like "man, I'd give Coraline a rimjob" or "That's a f**king stupid movie. I could do better on my Apple.".
If I were putting out a trailer, the last place I would want it to be is on Youtube with anyone able to comment. I would have done the exact same as them until I could control the placement of it.