I think the original assumption that "theft of a song is no more natural than theft of a car" is very obviously flawed even without going into what constitutes theft.
Humming a song you've heard is a very natural behaviour. Telling somebody a story you've read, too. Even trying to copy a painting somebody else has made; lots of people do that, and I don't think anyone would claim it was somehow wrong.
At the same time, accepting somebody else's property right for physical things is also very natural (if you don't believe, go ride a train and sit in a compartment somebody else already sits in - you automatically accept that the person has some amount of ownership over the compartment simple becuse he or she was there first. Or watch children playing; they don't know anything about law, yet they accept and use the concept of ownership).
So copying a song is much more natural to humans than taking away somebody else's car.
I don't often agree with Arrington, but I think he has a fair point here.