Looking at that from a publishers perspective...

That locks me in to your encryption system, and probably locks me into other things as a result. That also doesn't give me the ability to enforce other DRM things like only one playback per device, or only allowing it to happen between 7am and 7pm, it also means that i need to trust the client, whih is exactly what DRM is trying to not do (because if the publishers could trust the client, an HTML flag for "don't let them save this" would suffice.)

DRM by nature basically needs to be a black box. This just standardized how that black box hooks into the system, and sandboxes it to limit what it can look at (and so we don't have a repeat of the Sony music issues)