I don't understand, what other evidence do you need that these websites are being used for ddos attacks?

Cloudflare is not an official government entity. As a business you have the right to refuse service to anyone. Refusing these websites service is not the same as being thrown in jail.

The link in question.

https://booter.xyz/

> We are a "DDoS for Hire" company

> Our best layer 4 UDP attack methods is dns amplification, ntp amplification, ssdp amplification

You think they are going to amplify on servers they own?

Dozens of ddos for sale websites being advertised on hackforums being protected by Cloudflare.

http://hackforums.net/forumdisplay.php?fid=232

http://whois.domaintools.com/serverboot.com

http://whois.domaintools.com/synstress.net

http://whois.domaintools.com/kronosbooter.com

http://whois.domaintools.com/instress.club

http://whois.domaintools.com/clickboot.co

And countless more.