There isn't a technical reason it shouldn't be able to be implemented. Mostly it's a matter of getting the UI preference hooked up; it looks like they've already done something similar with the Flash plugin for users which wish to disable Flash, and this is a similar case.

The reality of the Flash situation is quite messy, with a ton of websites relying on nonstandard behavior and checks to see if Flash is even installed, and Google seems to have missed the use case for disabling the plugin entirely, rather than just blocking it. This seems like a similar oversight.

I'm torn here, as I see what Google is trying to accomplish with their settings spread. Mostly, the browser has chosen quite sensible defaults for the bulk of its users that needs the web to JustWork(tm) and they're hesitant to add extra checkboxes to avoid confusing folks. But with the politically charged nature of DRM, as well as the technical issues presented here (HDMI / HDCP is notoriously finnicky and the user should have more control over when it's enabled) I think a checkbox to disable the plugin entirely, similar to Flash, is the best solution.