hell with both "respecting community labor" and "but legal defense of fanfic" as explanations of why no monetizing AO3, tbh. the first one is important to get across for unrelated reasons, but clearly not making headway wrt this specific issue, and the latter often involves misunderstanding the legal situation.
(it's not that fanfic is only legal if it's free. it's that one of several factors for whether something is fair use under US copyright law is whether it makes money, another is whether it's transformative and OTW's stance is that all fanworks are inherently transformative. and many people are allergic to nuance, and nobody on any side wants to be the test case drawing black and white lines in this legal gray zone.)
they're a nonprofit. nobody gets to profit. people who benefit from the nonprofit's services especially do not get to profit from the nonprofit's services. users of AO3 are by definition people who benefit from the services of this nonprofit. or so @copperbadge says, at least, and he's been both working for a nonprofit and part of fandom long enough I trust him as a subject matter expert here.
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It's true that it doesn't get through to loud people, but I'm not sure it really needs to or that anything would.

