Well, copyright owners can still send DMCA takedown requests on links that include the decryption key. But Mega can't automatically take down copies of the data

Well, copyright owners can still send DMCA takedown requests on links that include the decryption key. But Mega can't automatically take down copies of the data (since any copies will be encrypted by a different key), and more importantly they can't offer big copyright houses custom tools (like MegaUpload did, and Youtube does) that find and flag material for them automatically.