It’s a free site to use, but not to run or to maintain, especially not with all these net neutrality battles.
If you donate more than a certain amount (pretty sure it’s more than $10), you can vote in their elections, because you’re considered a member, and that’s how memberships work. You can go onto the website, right now, and read it. Why are people still up in arms about AO3 needing donations to run? Their budget is publicly available.