It's not everyone for sure in my example here but I think it's because some love the thrill of the possibility of PVP anywhere and at anytime but only as long as they have the advantage. Don't underestimate gamers love to gank.
Cause after all, more WPVP would be happening on PVP servers if gamers really wanted it to happen. Small isolated pockets of WPVP is all blizzard has wanted us to do for a long time now.