Basically, population is too low to keep multiple queues enabled. Premades queue 3 bg's, can only join one. People are either slowpokes and don't leave queues as soon as they pop and only hide them (once inside a bg already) or keep the other queues up so they can start anew if they feel they are losing the bg. In any case, you get games that are temporarily 5, 6, 7..10 players in favour of a faction.
Surely this goes both ways and both factions have free games that start 13 v 2 for example, but why have this?
I mean, it's not really abuse, you could do that on retail too as far as I remember, but the bg would fill up before start there. And also you had less of a chance to have a 5 man premade randomly leaving because the other faction has more healers leaving you in an awkward 5v10 or whatever.
Other option would be to leave multiple queue as it is, but make it so that all queues are dropped upon accepting one, or paused (depends on what scripting difficulties each option brings)