Nope this is directly linked to ad revenues because StackOverflow don't care about them they can offer freedom. And if they ever get into financial difficulty you can count on me to subscribe to a paying "gold membership" and I still won't care if broken students get the same content for free and without login.
You never wondered why there is very few fake account on stackoverflow as compared to ad based social network?
Yep but quantity is not an ally to reach this goal, they look for people that have a good track record and are recognized by their peers. They don't care if the student that look for answers on SO is not registered.
Registering non participating members is only a trick to inflate your user base when communicating. What really matter is active users, and they will register don't worry.
You never wondered why there is very few fake account on stackoverflow as compared to ad based social network?