I'm sure control is the rationale for it, but I'm not sure the numbers play out the way they think in the long run. Modding by the community can fix broken games (leading to continuing sales) and maintain interest in the game for longer (leading to continuing sales). Imagine RTW without EB and such. DLC and expansion packs are still feasible with moddable games, especially if multiplayer is a focus.

But fortunately, MTW is easily moddable. So even an idiot like myself can remove peasants and ballistae, play unplayable factions, and tweak a few things to my liking. Now, if I could only get my hands on the .exe source...