Don't think it's a loophope, I don't even think DISCING is against the rules. Just against the general server 'nice guy' etiquette.
Personally I think youre overreacting. Yes, pball games are easiest to bs on, but its for a reason - they are generally hyper dynamic and all end within 2-3 minutes. Not much time is lost here...
It's also allowed to stay after the game is "over" on chop farms (with farms behind the woods), which may cost everyone a lot of time to sapp the way to them/get air... especially if the game ended early. so the "loss" here is even bigger...
It's allowed to ruin games as a watcher, which can also be very impactful and the general time loss would also be bigger here.
If we're talking stats and you don't want to take the 'unfair' loss here, I think it's only fair that we forbid 'unfair' wins as well - wins coming from watching, wins coming from AVing when the player lost...
To conclude, I don't think 'stats' argument is enough to make this any reasonable at all, and if we were to change it because of the "net time loss of all the players in game", a lot more rules should be added to maintain coherency.
Personally, yes, I am defending backstabbers and don't think anything should be changed regarding that matter. You just, errr, ban the dude next time.