I think with XF 2.0 there's an opportunity to fix this. There should be a section provided in the footer called credits (or whatever) where addon developers can put their copyright notices.
This will make life easier for style developers, webmasters and addon makers. Also it makes more sense because addon developers will be getting their copyright into the right place for site users to see it.
I've added an image below to give an idea of what I mean...