The answer is yes if you are flying coach in any of the DirecTV a/c (it's a little cheaper on shorter flights).

The fee is to activate the system, whether you watch the TV channels or the content (movies, etc.) that is loaded on the hard drive on board.

In the case of LAX-HNL, there is no DirecTV signal for most of the route, so all the $7.99 would get you would be about 30 minutes of DirecTV and then the on-board content.