Perhaps someone who's more versed in Apple tech can weigh in, but my limited understanding is that tvOS' users are mostly an illusion - the current user is just a flag exposed to running apps, and each app decides on its own what (if anything) they do with that information. There's no system-level separation of data or permissions for different users.
(In my experience, most non-Apple apps just seem to ignore user profiles on the Apple TV, and either behave as single-user apps, or have their own totally unrelated user profiles.)
Yeah, I think that's a assumption Apple made that most AppleTV devices would be in the same household where people can simply use the streaming app's ability to switch profiles. That sucks if you have roommates that pay for their own streaming subscriptions or the AppleTV device is in a common area such a dormitory. I suppose they could solve this by allowing ad-hoc profiles if AppleTV and user's IOS device is on the same network.
The user profile stuff as far as I can tell literally just determines the recently watched data in the Apple TV app. It doesn't even use your iCloud account when you select your account - I attempted to show some photos from my photo library on an Apple TV set up by someone else the other day and it just wanted to pop up their photos instead.
(In my experience, most non-Apple apps just seem to ignore user profiles on the Apple TV, and either behave as single-user apps, or have their own totally unrelated user profiles.)