I called this a while back. As soon as they started flagging clean hits up high and fining guys tons of amounts for the illegal hits I knew for sure this conversation would come up.
It's all stemming from Dustin Keller getting his knee entirely blown out. Here's the play.
That might be the end of his career. Think he dislocated it and tore 3 ligaments. He's closing in on 30 and I doubt he's ever the same.
There was a DB on Mike & Mike, can't remember who. Maybe a former DB, not really sure. He said that it's pretty much an unwritten rule that you don't go low on a guy who doesn't see you coming. It was one of those "you just don't do it, period" type of things that he said he had learned young and always played with. However, if a guy can see you're coming to make the tackle going low is a part of the game. He said that sometimes against TEs he'd go low just to put it out there that he may do it again if he has to get that guy to the ground in a similar situation.
The counter argument is that you'll have to potentially give up a first down if you can't get the guy to the ground by hitting him in the mid section. They had a good discussion on it (Mike & Mike is easily one of the best radio shows out there) and they said that a "strike zone" is coming because they aren't going to back off on going high and they can't let guys have their legs folded up like Keller's was. Their guest said he would have buried a shoulder right into his midsection and relied on bringing enough impact with him to knock the guy back but he did admit that he always viewed himself as a hitter so he wanted to put a good hit on someone.
Ultimately I agree with their guest. If a guy doesn't see you coming and can't defend himself, you don't go low. This play happened because Keller was just landing as the hit occurred so there was too much downward force for his leg get carried through like you would normally see and the result is a horrific injury.
They don't need a broad rule reducing all low hits. If a guy has turned upfield and can see you coming, it's fair game. Just add a stipulation that prevents knee shots to a defenseless receiver. Had the DB gone up high, he's flagged because Keller had barely touched the ground and didn't have an opportunity to defend himself.
You can't just say no going all together because you have some extra large guys out there who can't be stopped unless you chop them down. If all they did was amend the defenseless player rule, it probably wouldn't ever even be an issue.