The reason 20s are XP off is that max capped accounts, 60/70/80, and in this case, 20, were twinking and decimating leveling nubs in those brackets, well maybe no one did 80s yet, but that was the reason for it. 20s are an unintended consequence, but not one Blizz has any plans to rectify. 20s were never any good at PvP. They used to get destroyed by 29s.
The GY change was to stop FCs waiting around there and ideally reducing the duration of turtles. Of course people still use LoS by climbing on top of the tunnel entrance and have zero intention of capping, just delaying. If people can be cheapass pussies, they most certainly will be.
Blizz are trying to make BG experiences fun for everyone, that means F2P, P2P, leveling noobs, veterans etc. etc. Everyone. That was the whole point of XP off BGs to begin with. Of course with BC chanted heirlooms in leveling BGs, leveling noobs are unsurprisingly still getting smashed.
Blizz try to do a lot of things, and fail at most of them. They are still making a shitload of money on this thing, and if nothing else, I'm sure it will continue as long as that's the case.