Same for every video game that allows mods, custom levels, and so on. The only difference is that Minecraft users make no money at all for their efforts. How terrible! Poor children!
Either that, or people just enjoy being creative, making things is fun, and it's okay for kids to just enjoy making things and sharing them with each other, even if (oh, gasp!) the business that built the engine benefits from it. And being able to make any money from this at all is just a nice bonus.