I would have to disagree with you. You can't say that you have control just because you're on the ground. If you're standing and you "accidentally" clip someone in the back of the head, how is it any different from clipping someone accidentally when they are on the ground(Brock/Mir 1)? Brock was not intentionally aiming for the back of Mir's head, but he still hit it because Mir turned the back of his head into those punches.
I don't think it's that difficult to train for. You throw a hook and you try to extend it a little so you can wrap it around the opponent's head and possibly hit the back of the head. But of course this is only my opinion.
When you're on the ground and you're on top of someone, he turns to the side, you can aim for the back of his head or for the side of his head. You can almost always aim where you want to when you are on the ground and in control. As such, the rule states no hitting in the back of the head. The distance of punches is very small on the ground, which is why almost all back of the heat shots are on the ground, as your punches come so quickly that when you aim for a part of their head, that's what you hit. You can pick and choose where you hit on the ground.
When the fight is standing, you have NO control whatsoever on your opponent, as well as being so far away that by the time your strike reaches where you want it to go, he has moved his head in whichever way he wanted, and you take a chance of clipping him in the back of the head.
Your Brock example doesn't really work. Mir put his head in that position so Brock couldn't hit him, Brock would have to stop, get a new position/posture, and then hit more, as it is illegal to hit in the back of the head. That was Mir's strategy at that moment to control Brock's GnP, to put him in a position where he couldn't hurt Mir, as he could only hit the back of his head. The whole thing about that is the ref didn't call it a warning as he should have, that's why it's such a big deal. Usually there are a few warnings then a point, as hitting once in the back of the head when he is moving isn't a point taking move, but if you do it multiple times it is, as you have general control of where you hit on the ground.
Jones vs. Bonnar is the perfect example. He uses that nice spinning elbow to hit Bonnar, but it ended up hitting him in the back of the head. It wasn't illegal as it was in motion, you cannot tell where that hit was going to go, it's impossible to know.