Alot of people here are on the misconception of not being able to hit hard.
That isn't the problem, its players that purposely commit helmet to helmet or leading with the helmet contact on a defenseless player, period. That's it.
Game suspension is the only way to get the players attention to not "hit" someone using their helmet while in an undefended or compromised position, instead of using the actual tackling techniques.
Accidents will happen, but to purposely inflict a hit to where it can cause bodily harm to the extent of permanent injury or even death is not sport, it is just criminal.
Remember, this problem is not just in the NFL and in college, there have been instances of kids of various ages being paralyzed from contact as this.