Why should they do that? They're already -paying- for a license, and they're helping to support XenForo by offering services to reach a wider target audience (Galleries, CMS, Portal, Blogs done by 3rd parties will mean people will switch).
Perhaps a fee for posting paid software -on- XenForo (With strict rules regarding signatures and lite software), but to force developers or designers to pay further just because they wish to make some money off their work? Thats ridiculous, and would end up being the end of XenForo for me, no matter how much I like the system.