I am new to Tor and currently trying to understand the architecture of onion routing. However, I came across this TorDNSEL which makes it possible (for a tool using TorDNSEL) to detect whether traffic is coming from Tor exit node or not? This way websites can check the source IP against a list of IPs of exit nodes and block the traffic accordingly. I am not entirely sure why Tor publishes the exit node IP addresses publicly. Is it how Tor is designed or is it more of a "courtesy" to the websites/organizations who wants to block Tor traffic?
Any response would be much appreciated.