Apparently, the record labels have started paying radio stations to play songs as ads. It is illegal for record labels to pay stations to play songs. But, it is apparently legal for them to take out ads on the stations. Unless there are regulations on how much advertising a station can have within an hour of programming (and it’s possible that there are such regulations), it seems like the stations can just keep increasing their advertising slots per hour until most of what you hear on the radio is directly paid for.
By the way, this is once again why CDs cost so much.