It's also worth noting that OS X has been free as in beer since at least 10.9, and it looks like 10.1 was also free if you had 10.0.
For all intents and purposes, these wallpapers are free to the end user. Additionally, resold versions of the OS also have a cost approaching 0. Finally, the marketplace for resold wallpapers that were effectively free to the end user is nonexistent. Though there may still be interesting legal questions here, as you point out, the creators of the wallpapers almost certainly retain no rights anyway.
For all intents and purposes, these wallpapers are free to the end user. Additionally, resold versions of the OS also have a cost approaching 0. Finally, the marketplace for resold wallpapers that were effectively free to the end user is nonexistent. Though there may still be interesting legal questions here, as you point out, the creators of the wallpapers almost certainly retain no rights anyway.