Governments, contracts, civil and criminal litigation should work on a presumption of innocence (even if the bar is lower in civil court, it still exists), and require evidence (often an abundance of it, or so much it removes all shadows of doubt) to enact binding legal outcomes and punishments.
Groups, events and organizations should work on a basis of empathy and freedom of association. People generally don't want to associate with predators, abusers, and fascists (though the reactions of many commentators in hacker communities often make me question that assumption). And so many social systems are designed to filter those people out. And in more emotional settings like friend and support groups, believing victims is a way to exercise our empathy and provide mental and emotional support and healing for them. Which is the purpose of these groups.
This is like basic civics.