In hindsight it's kind of a wonder that the option to play the gametype(s) you want to play and ignore the one(s) you don't got completely overlooked.
That sucks, no doubt about it. I'd suggest playing around with your settings, maybe going for some of the cheaper system upgrade options if you can afford it.
As for the why you can't pick a specific warzone? Well that's a simple one. Queue times. On a heavy population server, it'll work. On the lower population ones not so much. As a staffer recently pointed out, you may like a certain BG, it doesn't mean everyone else will.
It's like when I use to play WoW, I could get into WSG's all day, but not an AB to save my life unless it was an AB weekend.