Referrer is exactly how you would do it.
The idiocy here is that most of these publishers are likely customers of Google's ad networks anyways, and the clicks through to the articles are yielding ad revenue to them, and they likely are getting analytics and tracking that identifies exactly where the inbound traffic is coming from.
It feels like a shakedown by people who are in other parts of the business totally disconnected from the web content / publishing arms, who likely know better?
