I programmed for a living and programmed in my spare time for either free or a pittance. When I played poker for a living, I also played poker for a pittance and for negative $$ (charity events). Hell, when i was in the navy and drinking for a living, i also drank for free when i wasn't working!
That's just my anecdotes, but you don't have to look too hard to see that most people are the same. It's a quirk of consciousness that we look at similar tasks as fun (and worth doing for free) and not fun (and needing compensation) based largely on context, and not solely out of consideration for what makes us more fit.