They’re essentially claiming ownership of the user submitted content by doing that.
Reddit reserves all the rights to everything you post ("When Your Content is created with or submitted to the Services, you grant us a worldwide, royalty-free, perpetual, irrevocable, non-exclusive, transferable, etc., etc.), but you alone are responsible for it (“Because you alone are responsible for Your Content, you may expose yourself to liability if you post or share Content without all necessary rights.”).