After spending about an hour researching, I've come to the following conclusion: these sites are an heir to sites like the old allofMP3.com of a few years ago. Basically, that site figured out a way to sell MP3s cheaper than amazon or iTunes thanks to some kind of loophole in some law somewhere. A couple years ago, some big labels tried to sue allofMP.com, but the charges were dropped. The sites went down by their own volition during that process, but since a bunch of have risen up.
Basically, these places shell out for this massive license that covers a lot of different music, and then you contibute to that license by paying $15. Technically, you're supposed to be even more careful about what you do with MP3s gained at this price for this reason, but my knowledge literally ends here. If anyone knows what's really up, I'd be more than appreciative. Google search does not seem to yield more results. Going by what this particular site seems to carry, though, I really wouldn't be surprised if they don't have the license for a lot of stuff.
That's about all I know.