This money is going to support writers. What's cultish about readers being generous? We knew it wouldn't be all readers, but we also know that there are a lot of people out there that do feel this sense of generosity to the people they read. This isn't specific to Medium. Patreon has this same dynamic where some people pay because they are required to, and other people pay because they like the feeling of patronage. Plenty of other examples, online and offline. TLDR; people are good and that doesn't make them dumb.