The Skeptics.SE site has a welcome message:
This is a collaboratively edited question and answer site for scientific skepticism. It's 100% free, no registration required.
All StackExchange sites have similar messages.
While it is true that you don't need to be registered to read questions and answers, but you do need to be registered to ask and answer questions - key functionality of the site.
e.g. when I click "Ask":
You must be logged in to ask a question on Skeptics
Log in below or sign up
This is true for several stackexchange sites, including Skeptics.SE, Politics.SE and Mathematics.SE, but not for others, such as Photography.SE or Poker.SE
The Meta site has an even higher hurdle of 5 reputation on the main site.
This has been a long-term problem.