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").
