Reggie Nelson targeted the knee of Le'Veon Bell. Bell caught the ball in front of Nelson. Nelson had plenty of time to gather his base, break down, hit on the rise, and make a great tackle. He did none of those things. Instead, he launched himself into Bell's knee.
Nowhere, at any level of football, is anyone taught to tackle by diving at someone's knees. The question that you can ask, then, is how do you tackle a larger man in the open field? Most coaches teach what's commonly referred to as an "alligator tackle". This means you try to get your head across the runner's body, wrap, and then roll. Feel free to google "roll tackle" and you'll see many examples of what I'm describing.
Those are the two basic ways to tackle in the open field. Gather your feet, widen your base, lower your center of gravity, and hit on the rise. The other way is the aforementioned roll tackle. As I mentioned, no one teaches launching yourself into someone's knee.
It is, however, a legal hit. To my knowledge, there's nothing in the rulebook that specifically prohibits Nelson's hit. It just makes no sense to try to tackle someone by targeting his leg. It's not good technique, no one teaches it, and the NFL has spent a lot of money over the past two years extolling the virtue of "heads-up" tackling.
We saw Antwon Blake make a textbook tackle on A.J. Green, causing Green to fumble the ball in what might have been the biggest play of the game. Playing with great fundamentals usually leads to good results. My problem with Nelson's hit was that the only logical reason to attempt to tackle Bell like that is by specifically targeting his knee. It's a legal hit,, but it doesn't mean I have to like it.
Nelson's antics post-game were certainly not endearing either. He seemed to have a problem with Mike Tomlin and continued to jaw at him while Marvin Lewis stood between them. He quit talking when Joey Porter appeared. I guess that sums up all we need to know about Nelson.