I'm not sure of the exact rules but they can't be much different than the ones for real world. (same production company)I remember reading once that they stopped putting a tv in the real world house purely for the fact that they would wind up getting countless hourse of boring footage of the housemates watching tv. By no tv or stereo, they were forced into conversations and outings for lack of better things to do. I bet it is the same here, that way we see lots of shots of the housemates talking about each other behind their backs. Hence good tv for us. 
For the purpose of the house here, I see it as a good thing. It would be so easy to be distracted by it and not do your work and just watch a marathon of some tv.