More precisely, Cloudflare should not be able to offer this service. AI agents should be blocked at the hosted API endpoint ("last mile"), not at a CDN or other sort of intermediary.
That said, if you're using Cloudflare Workers, where the endpoint itself is hosted by Cloudflare, that would be ethical.
If the CDN is implementing the explicitly-configured preferences of the specific site, what difference does it make if the blocking happens at the CDN vs. the site's origin server?