EME & User Agents

I came across this, and wonder about restrictions on Web Annotation
provisions.
As according to the article, EME is a precursor to 3rd parties bringing
restrictions on user agent behavior.

https://www.eff.org/deeplinks/2016/05/open-letter-members-w3c-advisory-committee

...
e.g.,
if the W3C defines a data-type, anyone can make a user-agent that can
receive and render that data. The people designing user agents might do
things that the people running the servers disapprove of (for example,
blocking pop-up ads), but that's not illegal -- so long as you don't break
the law, the company serving the data can't dictate how the companies
making the clients must handle it.