As noted in this other question and its comments and answers, the RaMBa"M allows one to derive benefit from the cooked mixture of milk and meat of an animal of a Kosher species that was not slaughtered according to Halachah, even though if it had been slaughtered properly, not only would it be prohibited to cook it with milk and to eat such a cooked food, but it would be prohibited to derive benefit from the cooked mixture.
Why does the RaMBa"M allow one to derive benefit from such a cooked food when the meat was not slaughtered properly (Neveilah)?