To me it seems like if the guy getting hit is standing then they never say anything about it. But if your on the ground and hit any where near the back of the head they constantly say to "watch the back of the head".
That's because when a fight is standing it's fluid, it's in motion, when you throw a punch and the guy moves his head to get out of the way and you clip him in the back of his head, he moved to the wrong spot. So, it's perfectly legal to hit in the back of the head when the fight is standing, as you can never say where it's going to hit.
On the ground, however, you have control of where you hit the guy, so when you hit him in the back of the head, it's against the rules, as you can prevent from doing so.
As far as fighters training to hit in the back of the head, that would be quite difficult seeing as how fighters move their heads to avoid strikes based on the angle, and you can't really train to hit the back of the head when you don't know which angle he's going to move.
Besides, it has always happened in MMA, and it'll always happen, that's part of having someone throw punches at your face and you trying to avoid them.