It's not semantics - placing a hand on someone's back isn't the same as pushing them. I can only go on what i saw, it really didn't look like a push, just the defender stretching out an arm and getting his finger tips to his back)..
And as i said earlier, if that sort of contact is deemed worthy of a penalty we'll end up with dozens of penalties every game. The ref deemed that whatever contact there was, was reasonable (it's not, as you suggest, that a push must always be considered a foul - there has to be degrees to this kind of contact).