I think perhaps you're looking at this from the wrong angle. A player has a choice (or actually a number of them) when making a tackle. In the first instance, the player can choose whether to make a tackle or not. Then he can choose the amount of speed, force and the manner of the tackle - all of which contribute to the momentum he ends up with.
While you are right that a player cannot suspend the laws of physics, he is not entitled to disregard them either. When a player initiates a tackle, one of the things he has to take into account, is the effect that his momentum will have. If he has imparted too much force and speed into his tackle so that he is not in control of his momentum and ends up committing a foul (potentially with a risk of injury to an opponent) he has to take responsibility for the consequences of his choices.
Taken to its logical conclusion, if the fact that the laws of physics prevent him from stopping dead after contacting the ball were an excuse for following through into an opponent then a player could just launch himself in the air with studs showing at an opponent's knees or waist and after seriously injuring said opponent, claim that he had done nothing wrong, because it was not his fault, it was the fault of his momentum.