In business terms, you could consider giving a cut to the modders a marketing expense (you might sell more because the mods increase the value of your game, perhaps?). But you can also consider it as providing a platform that modders exploit commercially, and you can squeeze them as much as is sensible in any commercial relationship.

I think the number of people buying a game because of a mod is rather small, so business people will see things more from the second viewpoint.