> But really, the NAT tracks should all be in the foo directory since foo is the primary artist.

I don't think that's true in general - it might also be that dot, dash and asterix are other famous artists that have contributed to the track or its a live collaboration or a classical recording or a genre in which it's not uncommon to have multiple artists perform a track (or the artists were just too lazy to publish the recording as a single, which we do save as they are credited :-) ).