> A jail broken/rooted device may not have the same protections since jail breaking is typically circumventing that sort of threat protection.

So your argument is that walled garden devices are less secure assuming there exist methods to convert it into a general purpose computer, which they empirically do.

Shouldn't this be an argument for such devices to be less trusted? After all, you can't always tell when this has happened (so you better assume it has), whereas as you say the devices designed to be operated under that threat model would then be more secure.