Momentum depends on speed going forward, doesn't it? If a player is running at 38 degrees to the touchline (where zero being straight at it), how is the ref supposed to calculate the momentum to allow? He doesn't even know the speed the player is running at to begin with.
So we get this situation where unless it's a howler of a forward pass or momentum is not involved, the pass is just waved on. However, what else can they do? Every ref will surely come up with a different opinion if they actually applied the momentum rule/law/whatever.