EME as originally conceived is about security for vendors, yes. But Firefox and Chrome, which take their roles as user agent seriously, made sure the spec is one that they can also implement securely on their end. This is a ridiculously huge improvement over NPAPI.

Browser plugins can restrict your freedom in two ways:

1. They can monitor and restrict what you do with the content you're accessing through the plugin.

2. They can monitor and restrict what you do with ANYTHING ELSE IN THE BROWSER OR ANYWHERE ON YOUR COMPUTER.

EME-restricted video can do 1, but cannot do 2. This is not a complete victory for freedom, but it's a significant step forward.

Most of us do not live in a free-software utopia where we have direct control over every part of our machine from the firmware on up. (Although I have a lot of respect for the people who are trying to get us there!) Even if we're running a free OS, we assume, against our better judgment, that the firmware is doing only what it should be doing. If it's not, then it throws the rest of our freedom out the window.

Sandboxing EME allows us to ensure that one non-free part of the system, if we can't get rid of it, is restricted to doing only what it says it can do. This is a lot better than the status quo for any mixed free/non-free system (including free OS/non-free firmware). It's not quite the same as getting rid of it or making it free, but it's certainly not nothing.

A major issue is that arbitrary code can do arbitrary things, up to and including installing rootkits on your machine (for example, the Sony rootkit debacle). The sandbox means it can only do what the sandbox allows it to - in the case of Firefox's sandbox, that's very little.

"I should be able to watch whatever content I want on my system without installing proprietary software, even if that software is highly sandboxed and can't even see anything on my system and can't cause any permanent changes" is a political and moral issue more than a technical one.

Or use a free and open source browser that doesn't implement it. If this is something people want to take a principled stance against, those browsers will doubtlessly continue to exist.

Unless you're referring the content access. That's a general trend with technology that likely won't be stopped. If you choose not to use certain technologies, it's unsurprising that you won't be granted access to certain things.