[alexkus]

Then it's easy for law enforcement to force Mega to remove all known versions of certain copyrighted material as they can now prove that Mega are hosting copies of that particular bit of copyrighted material, .e.g.

If they find a ripped version of a movie/ebook/whatever they can just encrypt it using Mega's scheme (which would now derive the key from the data) and get a single version of the file out. They then tell Mega to remove any files that match that encrypted file.

If all files are encrypted with a random key there's no way for law enforcement to do this.