It's against the Cloudflare ToS:

> The Service is offered primarily as a platform to cache and serve web pages and websites. Unless explicitly included as a part of a Paid Service purchased by you, you agree to use the Service solely for the purpose of serving web pages as viewed through a web browser or other functionally equivalent applications and rendering Hypertext Markup Language (HTML) or other functional equivalents. Use of the Service for serving video (unless purchased separately as a Paid Service) or a disproportionate percentage of pictures, audio files, or other non-HTML content, is prohibited.

It’s okay at least for serving images (as long as you use workers): https://news.ycombinator.com/item?id=20791660 from CloudFlare CEO. It’s not clear to me if piping terabytes over terabytes of, say, opaque data would violate anything.

This is a clever exploit none the less (not a pejorative).