I believe Clearchannel changed their name to IHeartMedia as well. It's all Clearchannel stations on the app, which is probably why/how you hear it often too.

iHeartRadio is in fact a streaming radio/music app, kind of a mashup between Pandora, and traditional internet radio.

iHeartMedia (the parent company) is a quasi-new conglomerate buying up radio stations all over the US, which is why you hear their advertising all over the place.

I think it has to do with different stations that are not Iheartmedia (formerly clear channel) For example, a CBS Radio station here in Los Angeles is KROQ, Which is an Alternative Rock station. They are able to be listened to on Tune In. Their direct rival ALT 98.7 is an Iheartmedia station, which is blocked on the Tune in app.