They add CORS support to their server resources and get the false assumption that their resources can only ever be retrieved from a web page under their origin or an origin they have trusted (via CORS). They believe that this also protects them from malicious users making requests from outside of a browser (eg. via curl).
CORS is not security, it is loosening of security (namely SOP).
The number of times that I have had someone ask "why can someone access the endpoint via curl? I thought this was protected by CORS" makes me sad.