Playing the devil's advocate here:
From a business sense, pay-for-play is like advertising, isn't it? It is a way of building an audience.
In the end, there are audiences that radio stations have built up, and pay-for-play may be cheaper than plastering the streets with posters, or paying promoters, or touring, which is just as expensive.
One equivalent in the search engine world is SEO vs PPC listings. Each has its cost. If you didn't know the people looking for a particular genre of music, but somebody did, and they did it as a business - i.e. quit their jobs, put in risk capital, why wouldn't you pay them for the information?