EE's business model was built in the early days of the internet, and before SO came around they had lots of paying customers. "Tricking people to sign up" was really just their attempt at monetizing content. It clearly did not pan out to be the best long-term strategy, but when you already have thousands of paying customers, it gets pretty difficult to "open up" to the general public vs. a company like SO which started open and monetized in other ways (Careers).