Before cloudflare, it was difficult to run a DDoS-for-hire service because competing services would all DDoS each others' websites. Back when CDNs were all "call for pricing" affairs.
Cloudflare had the insight that the more DDoS-for-hire services there were out there, the greater the demand for their services. Offering free DDoS protection to DDoS-for-hire services helps keep customers coming back for more.
> Before cloudflare, it was difficult to run a DDoS-for-hire service because competing services would all DDoS each others' websites.
I mean, you don't need websites to advertise. Most DDoS-for-hire services back before 2009 advertised on IRC, NNTP, via ads in .NFO files found in warez releases found on Kazaa and BitTorrent, and so forth. (Some of the very tech-headed ones ones had Freenet sites.)
Cloudflare had the insight that the more DDoS-for-hire services there were out there, the greater the demand for their services. Offering free DDoS protection to DDoS-for-hire services helps keep customers coming back for more.