You only see what is inconvenience to you.
Alliance is known to be more into ganking than Horde is. Alliance ganks way more than Horde, but that still doesn't mean Horde wouldn't do the same to some degree.
You don't really get to see your own faction camping opposing faction, unless it's common place for both factions, like Stranglethorn Vale, Dark Portal, Tanaris, Un'goro, HFP, Ring of Blood, Zul'Drak, Storm Peaks and Icecrown. The quest hubs on other zones tends to be away from other faction's quest hub. There's some overlap, but not much. So ganking can't really be seen/witnessed unless you go to opposing faction's quest hub area.
Dark Portal is known place where you usually get ganked, so is Thrallmar (and Honor Hold), how do you not know this? Tarren Mill has tradition of being spot where you can see Alliance player killing Hordes, I don't know why, but it happens frequently.