True, but Apple has to pay for maintaining the apps store, curating apps, localizing the store, etc, so it's not exactly free of costs for them either. I just do not understand the notion that Apple should somehow let app developers have access to the app store free of charge. 15% is not an unreasonable markup; nor 30% to cover the costs of initial review. Even so, for many apps I doubt the money they get covers their costs. If Apple charged a fixed fee for access and review many of the apps currently on there would not exist.
Every apps on the App Store, even free ones, must pay Apple at $99/year to maintain listing. Enterprises must pay $299/year. The exceptions are charity apps and government apps.