Why does the RaMBa"M allow one to derive benefit from cooked mixtures of milk and meat of improperly slaughtered animals?

Question

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)?

Answer 1

The RaMBa"M considers this point to be rather fantastic - and even says so.

The underlying logic is as follows:

There is a concept in Halachah of "אין איסור חל על איסור", which means, "a prohibition cannot apply to another prohibtion." In plain English, this means that, once something is prohibited, additional categories of prohibition cannot be applied to it.

Eating improperly slaughtered meat from a Kosher animal species is prohibited. As a consequence, the prohibitions of meat and milk do not apply to that improperly slaughtered meat. The RaMBa"M takes this to mean that the entire category of meat and milk prohibition cannot apply to this meat at all. Therefore, even though there is no prohibition against deriving benefit from improperly slaughtered meat, any additional prohibitions on the meat (such as deriving benefit from it if it is cooked with milk) cannot apply to it.