The point of Cloudflare is that hosting your website for free is a service, and you "pay" for that service in strengthening their network; from their S-1[1]:
> Free customers are an important part of our business. .. Our free customers create scale, serve as efficient brand marketing, and help us attract developers, customers, and potential employees.... In addition, the added scale and diversity of this traffic makes us valuable to a diverse set of global ISPs, improving the breadth and economic terms of our interconnections, bandwidth costs, and co-location expenses.
Your theoretical cost as a free customer is outweighed by the positive effects of protecting over 10% of websites on the internet, largely for free. Now, if you run a free video site proxying all your multimedia stream segments to serve pirated movies, or create a site dedicated to user image uploads and quickly balloon to serving hundreds of terabytes a day at the expense of Cloudflare, your value to the network is trumped by how much you're actively costing them in uplink costs and risk exposure.
0: https://community.cloudflare.com/t/the-way-you-handle-bandwi... (they were suspended for proxying over 140TB of a bunch of archive files/binary files over the course of 15 days)
1: https://gist.github.com/judge2020/e49138d588950167b736c630aa...