Cross-checking, in the back

Can anyone explain to me why at all levels players insist on cross checking their opponents in the back while referees let them get away with this? An example would be a midfielder coming down a side and the defender is driving the offensive player away from the goal with a push. The push in this case is against the back of the offensive midfielder with two hands separated and the cross makes obvious contact with the back of the opponent.
I was always under the impression that you had to have both hands together if you wanted to initiate a push and it has to be in the front or at the very least, the side.
I was always under the impression that you had to have both hands together if you wanted to initiate a push and it has to be in the front or at the very least, the side.