I don't think it's ad revenue -- they say on that page that "Visitors are allowed 3 free articles per month (without a subscription), and private browsing preve

I don't think it's ad revenue -- they say on that page that "Visitors are allowed 3 free articles per month (without a subscription), and private browsing prevents us from counting how many stories you've read. We hope you understand, and consider subscribing for unlimited online access."