May not be accurate anymore - although I used to work closely with people at BYUtv. At the time, sign in was required to allow us to acquire some of the content that we broadcast on our linear channel (i.e. Disney movies). In order for them to agree to allow us to stream their content on our channel online, we had to prove that "subscribers" signed into the platform (similar to other TV viewing apps where you sign in with DirecTV, Comcast, etc.)
It's a minor technicality, but if it wasn't for that content, I don't think BYUtv would care about you signing in. Hence why you can watch all the on demand stuff you want without being requested to authenticate.