They do platforming (with community support for forums and comments), storage, and CDN all in one - for free (to mod developers). They have to monetize somehow, and it's best they charge now than offer a free service only to pull the rug under people once they're on the platform. Nothing is stopping you from putting your mods on, say, GitHub. Hell, if you're putting together a modlist you can use wabbajack and it automatically downloads stuff from GitHub.