That makes sense for posting, where Tor users often get a CAPTCHA etc, but I can't even read it. If I go to any site on SE, I'm hard blocked, I can't even fill in a captcha to bypass it.
Loading CAPTCHAs is still very expensive, and is not an answer to large-scale DDoS attacks. Those have to be handled higher up the stack, usually at the proxy/load balancer level.
