You misunderstand the goal, which is to prove that an unlicensed copy is stored in the system.

It's like a scenario where Alice sent another Bob some highly illegal data that's a crime to possess, and encrypted it with Bob's public key. The police got a warrant and have access to Bob's email but not his secret key. They can prove he has the data (and convict him) if they have a copy, encrypt it again with his public key, and compare the files and find them to be identical. Could they have gotten the unencrypted from Bob's email? No, but they didn't have to.