I'll be honest -- I've used both MEGA and Megaupload for piracy, and MEGA makes it easier.
This is straight up wrong. Mega cannot decrypt files and search for copyright files just like that. They need to have the key for that. I have shared a lot of copyrighted stuff on mega and the links are still available publicly but in closed forums( still active ). mega would've taken it down a long time back if they knew what was in it. They can only take down files when corpos notify them with the base links to the file. If they don't it stays up.
OR
You uploaded copyrighted material to your own computer to your own cloud, downloaded it and the file got taken down ? This is not possible. If yes then I am in trouble.
How would MEGA do this, without crawling the web looking for links + keys? Which is certainly something they'd have no interest in doing.
Naturally, I'm sure that third party "rights enforcement" companies might trawl for these links and look for infringing content and issue a takedown notice to MEGA, but there must be an ongoing arms race between people sharing links+keys and the bots crawling for them.
By doing that.
They receive reports and take links down.