There's a difference though. DRM music files are sold directly to consumers. These consumers have to be able to decrypt the files on their own computers / devices which makes DRM useless.

The movie files in this case are not meant for consumers. They're only decrypted on specialised equipment not accessible to the general public. If you manage to get your hands on a DCP file there's nothing you can do to decrypt it without access to the specific projector system it was intended for (unless someone screwed up somewhere of course)