My understanding is the racial and ethnic categories are what is the problem. If you're buying 25-34, you can't exclude the Urban station. (Facebook is having their own controversy with their self-service ad tool; real estate agencies excluding minorities from its targeting). David can probably give a better explanation than I can.
I can see it with real estate advertising, but it makes no sense to me that advertising for, say, country star Jason Aldean's tour can't be pitched to 25-34 without letting the agency know that it shouldn't bother with stations running urban and Hispanic music formats. I agree, I'm probably not getting something here. Maybe David will drop by to explain better.