If I'm paying for a video channel, I expect not to have to pay for it again by suffering through advertisements!
Also I would be afraid that if NASA-TV-2 (Your Mars Information Station, broadcasting 23.5 hours a sol!) was advertisement-driven, it might start to affect the mission the way it does, say, a hockey game (Okay, on the next break in the action, can you guys just stop the EVA for three minutes while we run a commercial break?)
The pay-per-view idea is a good one, though. Sure you could set up a massive satellite dish in your backyard to pick up the NASA-TV downlink, but for a mere $10/month we'll add it to your cable lineup. Yeah, that's good.