When hovering over the "protect" link underneath a question (15k+ users), the
title text reads:
disallow answers by anonymous and very new low rep users
Isn't the anonymous users part irrelevant, as users must sign up to post on the site?
No, you have to sign up to ask questions on Stack Overflow.
Unregistered users are still allowed to answer them (unless protected, of course).
Users must sign up to post questions, but they can still post answers (to any question).