To realise the true answer to this, you need to take off your twink hat and put on your business hat, so to speak.
It's quite clear that Blizz wanted to segregate twinks from regular levelling players because the twinks were destroying everything in their path, and therefore destroying the expereince of potential new subscribers who were still sizing the game up. There can be no doubt that a number of new players entered a bg, got stomped on repeadely by twinks, and thought "Screw this. I don't like this game anymore. /unsubscribe /uninstall". That's not something Blizzard wants, since bleeding new players will lead to a sizable loss in revenue. Blizzard may not care about balancing low level pvp, but when it comes to losing new customers, they always care and so do their shareholders. Something had to be done, so they split things up which imo was better for everyone, including twinks.