For example, SO copycats are legitimate in that they respect the license and otherwise just serve the content to whoever sends them an HTTP request. As far as I know they don't spam links to their domain anywhere. They are low-quality and of dubious utility for sure, but I'd rather not make the Internet a place where you need to prove quality & utility to someone to be able to host an HTTP server.
The real problem is that a dumbass like Google comes along, sees this and decides that it should rank higher than the source content.