[heleny]

HBO gives you free advertising

That may have serious problems with FCC laws.
Broadcasters must offer equal time advertising opportunities to opposing candidates, at a similar time slot, for the same price.

[codebreaker]

Good point they would have to advertise for all the major candidates.

[general_re]

HBO isn't an over-the-air broadcaster using public bandwidth - it's a subscription service transmitted via satellite/cable. I would venture to guess that they probably aren't bound by those laws.