Some of them are designed for having people buying upload credits. I'm into one where only the admins are allowed to post new torrents, and they keep everything on 5 seedboxes. It's almost impossible to seed back, as their own seedboxes are pushing too much upload, then old torrents are removed and re-uploaded "to gather interest", but that means you will never find new peers. And then they always send messages complaining that they're spending 500 a month for those seedboxes "to guarantee fast downloads" and everyone should become a donator or the site will close. Assholes, those seedboxes are indeed guaranteeing fast downloads, but also are guaranteeing zero upload back