From the Minecraft EULA:
Any tools you write for the Game from scratch belong to you. . Modifications to the Game (“Mods”) (including pre-run Mods and in-memory Mods) and plugins for the Game also belong to you and you can do whatever you want with them, as long as you don‘t sell them for money / try to make money from them. We have the final say on what constitutes a tool/mod/plugin and what doesn‘t.
As such, we will not be allowing Paid/Premium Plugins/Mods on any Sponge Platform, as it directly violates the Minecraft EULA.
Developers may continue to earn money through donation services, but cannot create any exclusivity in who can access their Plugins/Mods. This is as doing so is effectively putting their software behind a paywall, which would violate the EULA.