Legally speaking, do political candidates really have to purchase the rights to the songs that are played at rallies, events, etc? I honestly don't know. I would assume they do for TV & radio ads, but it would seem to me that "background music" for an event or a gathering of some sort would be fair game. Then again, I'm not an attorney.
I'd say if they're not required to by law to purchase the rights, then Mellencamp has no real argument. However that shouldn't keep him from asking McCain to stop if he feels that strongly about it. And that's really all he's doing. It's not like he's raising a big stink (not yet anyway).