But that's not the same thing as having an EULA for the entire device as a whole, including its local functions that don't rely on a connection to Roku servers!

The EULA is for the OS, not the physical hardware. So all it takes is them updating the OS for "security reasons" and they can sneak a new EULA into the deal, locking you out of using that OS on new versions if that's what they want to do. Unless you flash a new OS to your TV you're stuck using their software and following the rules of that software.

And there's really no way around that without really hurting legitimate software licensing situations other than maybe making it literally illegal to have devices ship with Auto-Updates enabled, forcing user consent to update anything, which sounds like a great way to piss off the general public.

Hmmm.... Maybe legally all devices must be flashable easily without removing or modifying physical bits of the device? That way if an OS Update goes a way you don't like then you can flash an old version or DIFFERENT OS entirely onto the device you own, regardless of if it's a TV, phone, microwave, whatever