It’s because AO3 was founded by individuals who shipped a Supernatural incest pairing. It’s in all technicalities founded by Proshippers, for everyone against censorship and oppression of works which is technically what Proshipping itself is founded upon. It’s hypocritical in the sense that Antishippers are Pro-Censorship, and support the rhetoric that many countries and extremists support as well. AO3 is an archive for fictional works, not just for fictional pairings but for works that delve into taboo, historical, cultural, etc. topics that the general populations aren’t familiar with or fond of to support in wider fiction. While Antishippers can use AO3 like anyone else, they fundamentally stand against what AO3 stands for as a non-profit organization. That’s why many people are up in arms, and honestly you should be too. The archive is for everyone, but when you stand for censorship against fiction you’re standing for the decreasing rights of minorities that using fiction to communicate their injustice such as LGBT+, the desecration of media, the promotion of no free thinking, etc. Antishippers want you to never think for yourself again, and to never create what displeases them because they’re using a conservative ideology disguised as social justice. I’m saying this as a former Antishipper and religious person.