Can you explain what parts of SteamOS are not controllable in a way that makes it more restricted than Arch, which it is based on?
Valve won't release the source code and I don't use it, so it's hard for me to tell which packages are proprietary and which are not. Steam client for sure is proprietary and it comes with the OS. Arch by default is Free Software (other than proprietary blobs in the kernel) and you can audit what each program does and modify it. With SteamOS you can't do that, because Valve keeps secrets from you on your own device.
[If the account owns the game - allow user to download and run the game] is a DRM sure… But it’s kind of fair, no?
To play any game you have to install and run the proprietary Steam client and be logged in to an account. Even to play singleplayer games. Even if you bought a physical disc. There are stores that don't do this: gog.com and itch.io. They provide an optional client for convenience, but you can just download a game's installer from the website and install it on any PC any time you want. In case of Itch the client is Free Software so anyone can see what it does and modify it.