I'm not sure there's any working DRM for movies currently. DVDs and BluRay can be copied. Streams as well. DRM for DCP "works" but no one was copying 35mm film either - and the old analog hole is still open. All the DRM makes it more complicated to master and distribute legal copies -- I'm not convinced it reduces copying.
The DRM for DCP lets you audit who played your movie. It's not that expensive (I think $7500 or so plus a small per-copy fee), but if a film gets any kind of release at all this is a lot cheaper than paying someone to get on the phone and call around to every theater that booked it to get a written report of how often it was screened etc.
It roughly doubles the price of the digital projectors (granted, a more-or-less one-time fee of an additional ~13k USD). If auditing was all that was needed, I'm sure you could just tag the digital audio track of 35mm as well (or any non-drm'ed digital format).
Yeah, but a lot of the digital projectors for existing theaters were installed with the aid of financing from the major studios on pretty decent terms for the exhibitors, plus (I've heard) you can get away with paying the projectionists a bit less because they don't have to do as much.
The thing is that striking a copy of a 35mm print would have been a) slightly technically inferior compared to doing so from a negative and b) costs a lot, like $35k, so while there was a little 35mm print piracy it usually involved shenanigans like fraudulent damage claims or somesuch.
With digital distribution, you're giving people a copy of the film in 4k which is as good as the state of the art in Hollywood a few years ago. If you didn't have all the DRM on there the incentives to make a copy are almost overwhelming. It doesn't even need to be a perfect copy, ProRes is so good that most consumers wouldn't notice the difference.
