Then explain why it does it.
For me it does not.
The law of conservation of momentum is pretty simple.
You have a boat, throw out a stone to the back. The sum of the boats momentum plus the stones momentum before the action and after the action is the same.
Now, why do you believe there is no other way that the boat can gain momentum? The "law" certainly does not even cover this question and this engine ;D
And if you would kindly read up the theories about it you would figure that virtual particles take the "other part" of the momentum ;D So it is even covered by the most basic variant of that law.
However, it would be cool if the english speaking world would follow the rest of the world and would stop calling basic laws like the law of conversation of energy and momentum "laws". They are no laws of physics, they are axioms. Perhaps that would make teaching them in school more easy.
E.g, all "laws of thermodynamics" are in german simply "axioms" and not laws. However there are plenty of proven laws of physics and it is important IMHO to distinguish between laws and axioms.