They included that in their reply:

ClearChannel (and therefore iHeart) sells advertising enmasse so that messages and products can be pushed the largest groups. They hype up the music festival like its a great big deal, and then they give away tickets.

Basically, the bands and holding the event are cheap (the bands selected are not in high demand and thus cheap booking) in comparison with the profits of hosting a subliminal ad show with a (happily) captured audience.

Note that this is my interpretation of the other poster's message I thought I'd chime in with, I have no first or even second hand knowledge on this topic.