You are free to implement whatever content protection systems the developers of the content protection systems allow you to implement, so long as it doesn't conflict with their contracts with content providers (which it probably will in many cases).
The HTML5 ECE spec is intended to make sure it's a criminal offence to implement any "content protection" scheme without permission. In the eyes of both W3C and its proponents, that's a non-negotiable feature.
Because if you don't have a permission to do so, you're basically 'circumventing' DRM which is illegal under copyright laws in many countries around the world.
This proposal only defines how the browser communicates with the module. It does not define, how the module communicates with OS and hardware. These modules will use OS facilities like Vista's protected path. If your OS does not support it (and free software OS like Linux cannot support it by definition), good luck getting it. And even if it supported anti-features like that, the owner may not bother with porting ("not enough market share for you").
The HTML5 ECE spec is intended to make sure it's a criminal offence to implement any "content protection" scheme without permission. In the eyes of both W3C and its proponents, that's a non-negotiable feature.