For a LONG time it WAS a penalty to actually assist a runner through pushing, pulling or physically carrying them forward. It was only in the last 20 years that the NFL eliminated the penalty for pushing, although pulling or carrying remains a penalty. Why they changed it, I don't know.
en.wikipedia.org
I think they need to go back to it being a penalty. The concept has been bastardized. Not only that, but it results in a bigger injury threat to linemen on both sides of the ball. It's one thing to get pancake blocked or even double teamed. It's quite another to be plowed under by four or five guys. Back and neck issues like we saw with Jones in the SB will continue to become more commonplace. For a league claiming err on the side of player safety, the Tush Push is a blind spot.