I don't think we watched the same video...
because if that was a pass, and the RB was in the act of catch it and Reid had made that same hit, it would have been a 15 yard penalty.
Reid lowered his head, and exploded through the ball carrier, just like they teach a 6 year old to make a tackle in Weight Ball, just like they teach a Freshmen in High school, A Freshmen in College, and yes, even a Rookie in the NFL how to make a tackle.
I think its bullshit that if he was doing that while a receiver was making the catch, its a penalty, because its football, not fucking soccer.