Equating forum bans to game bans (in an apples-to-apples way) is impossible, since you can't really ever commit exactly the same set of infractions just in the forums as you can in-game. Therefore, being banned from one 'side' cannot logically ban you from the other in an equal amount.
That being said, if someone commits a repeated or heinous enough offense, there's no reason they shouldn't be banned from the other at some certain level.
For instance, if you get a weeklong ban from the game b/c you exploited a bug against the ToS, you may not have done anything wrong that someone can do on the forums, such as flaming, death threats, stalking, etc. In that case there may be no justified reason for banning them from the forums. You might also have repeated crap postings on the forums and gotten a ban there, but still have game access, because you only committed offenses on the forums themselves, rather than repeating whatever in trade chat.
Banning someone from both should be reserved only for those who are concurrently breaking the ToS in both areas, or if someone is banned from all Blizzard services as a last resort.
I get what you're reaching for here, but what you're also unintentionally doing is attempting to say "There's two different communities." There aren't. There's one community of WoW players, another community of SC2 players, a third of D3 players, and they sometimes overlap.
The goal here is to clean up these forums, as far as I'm concerned, and if we happen to lose a few of the most egregious trolls' subscriptions as a result of them finally being held accountable for their behavior, I also consider that a net win.
A forum ban alone isn't a threat to them, so they'll continue the behavior. If it hits them where it hurts, in their ability to play the actual game, that's another thing. They'll pay attention to the consequences.
And an in-game ban equating to a forum ban... well, since you're supposed to have an active WoW account to post in the forums, if you're banned, your account isn't functionally active at that point, and you shouldn't be able to post here until it is.
I really think this is the most sensible solution to our rampant trolling problem. I'd love to see it implemented over on the Diablo 3 forums more than anywhere else, but believe me, the WoW forums can at times approach that level of vileness, and that doesn't benefit anyone other than those who get a twisted sense of satisfaction from irritating others.