They provide a platform with fine support for mods. That's some extra work and cost in the development of the platform which allows modders to spend less time and money to finish their mod.
I agree the percentage is ludicrous, but I don't find a small fee unreasonable at all. Developing a customizable system is hard. We may argue that they may sell extra copies based on that feature, but we'd never have objective data to back up or refute that claim.