Local hip hop radio Djs or hosts, are u required to play these same songs(often watered down) songs daily?

i turn on the radio and all i hear are the same repeative songs...for example they keep playing that Pitbull and J Lo get on the dance floor song every 20 minutes, and that Black Eyed Peas just can't get enough every 20 minutes. I apprechiate that what guys do and i like being entertained it must be a fun job. But, why is it neccessary to play these songs over n over again. Is it the listeners that request these songs but what if they want to listen to Tech 9 or Nas or w/e can they do that? or are u forced to reject any songs not on the playlist.