Imo it's not really a double standard because it's pretty different and distinct communities that don't mix that much. ~Fandom as a whole~ has a lot of different standards depending on who and where you are.
Speaking as a fanartist I do get the impression it's mostly the writers and fic readers themselves that enforce the 'no fic commissions' rule as a community standard. Also it should be said that this topic is not wholly undisputed among fanartists either, of course, though the general consesus seems to be 'it's your risk of having to give share of your profits to a copyright holder'.
The reasons for the differences might be found in the specifics of copyright law application for words and images respectively (though I'm a bit disinclined to give fandom that much credit), resulting in more fanartists being willing to take the risk, and also in how the respective industries work outside of fandom. The publishing process for the kind of fictional prose that fanfic is doesn't involve a lot of commissioning (or at least I believe it is more submission based but I'm not a writer), however for illustration - and almost all fanart is illustrative - it is the norm.
I'd also guess that in fic circles the risk of copyright lawsuits might be more seen as something that would impact the community negatively and not so much the individual author? For historical reasons, maybe? Input welcome.