Khan vs Peterson, Ali vs lyle

I didnt take the khan peterson fight personally cuz im not a fan of either guy, that being said, i really enjoyed the fight and dont think there is a loser, it was a close fight and could have gone either way. But i came across a fight the other day, Ali vs Lyle, and Ali kept pushing lyle back without issue, and i have earlier stated i have seen pushing alot in boxing and most refs dont say anything, so the question i have is is this pushing thing a new rule, or has it just been ignored by most refs as not a huge violation?