A. The wisdom is that Allah has forbidden us to use fire on His creation. As for the rest of his orders and prohibitions He doesn't need to explain Himself.
However, if it were my parents I would do my best to persuade them to change their mind. The body continues to feel after the soul has left it. The Shari`a recommends for us to wash it gently, not roughly, and to use lukewarm water, neither cold nor hot. Are they sure they will not feel the flames? As for what happens after a non-cremation burial, then that is the way of the Prophets. Do they not trust that it will be better for them also? It would greatly distress me if my parents committed this act on themselves, no less after their death than if they were alive. If they expressed their intention to have it done nevertheless, then I would express mine to not witness it, short of attempting to stop it if I had the power, although it would not be incumbent upon me to stop it since I am not responsible for them. Allah knows best.