In the past (and often still the case), if there was a problem originating from an IP, an untrained law enforcement officer would track down the IP, find who owned the IP, and contact them about whatever activity they were investigating. The IP owner would respond that they are a Tor exit node and it is not their responsibility. Neither the Tor ExitNode Operator nor the LEO would want this conversation to occur. This process is time consuming for law enforcement and would put a bad taste about Tor in their mouth; creating an us vs them between Tor and authorities.
Now the Tor Project does outreach to authorities to educate and show them how to find if an IP was an ExitNode at a given point in time so they can speed up their investigation without focusing questions and harassment on the Tor node operators.
Furthermore, there is a file that your client downloads at regular intervals called the consensus document that contains the entire list of ExitNodes available on the network including what their exit policy is. Your client will make connections to an ExitNode based on their exit policy and what type of service you're attempting to connect to (e.g. HTTPS on port 443). In essence, everyone has access to this list whether they know it or not. Building a web service around it doesn't really change anything.