So... I can technically implement my own EME standards compliant web browser, but that browser won't actually be able to interoperate with the existing set of websites that make use of EME. What's the point of the standard again?
Certainly complying with EME isn't enough for web compatibility, but EME plus any given CDM isn't in reality interoperable either: sure, Firefox and Chrome both use a Widevine CDM, but Safari and Edge do their own things. As such, in principle a website could easily not support Safari and Edge (given you get the two larger browsers with one CDM).
To be fair, you can make a similar argument about what's the point of a standard for the video element: in reality, you need to support H.264 encoded video, so just supporting Ogg/Theora/Vorbis (as some early implementations did) doesn't suffice, so what's the point of that standard? (Also the img element, the object element, etc.)
But yes, EME is different because it fails to fulfil its use-cases in a fully free implementation (one can imagine, potentially in the future, a free software implementation that passes encrypted content to a hardware module that implements the decoding, but that seems like little gain and unlikely to happen).
