For your mind I dont think a TV Is a screen with moving dots that form an image, you'd have to think pretty complex in your dream for that, for your mind its an actual window to whatever happens I think... so somehow you become immersed with it. But when you watch an exciting movie, you can get totally immersed into it, right?? Perhaps its the same thing but amplified, since the dreamworld is unstable and when you're concentrating on something else it happens and the other things fade.