I never really thought about it before, since I don't write much fic, but it is a pretty crazy double standard. A commissioned piece is a one-off, not a mass production, so one would think the gray-area guidelines would be the same for both.

Maybe it's just that fanfic falls under greater scrutiny because of publishers? Or because fanfic is so easily reproduced and distributed? (whereas with fanart, distributed images are too low-res for print) Or because of wank? I dunno.