If the users were using free domains instead of paying for a domain do you think they'd use paid cloudflare? The cost of a domain is so much lower than the cost of Cloudflare.
I could at least imagine a scenario along the lines of: penniless college student creates a site at a .tk domain. Later, the student gets a job so he is no longer penniless, and meanwhile, his site actually becomes popular, so he signs up for cloudflare, maybe even registers a .com domain, but keeps the .tk domain alive because that's where most his traffic is coming from.
Not sure how common that is. But I don't think it's a given that all sites hosted on .tk domains are unwilling to pay, especially not if you consider that they must be somewhat popular if they need a CDN.
(The sort of personal homepage that most of us had back in the 90s would never need a CDN because it would get 5 hits per week.)
I don’t know and that’s why I’m asking. Not paying for a domain is not a reason enough to expect not paying for cloudflare - these are different services. Also note that even not paying for cloudflare is not enough - I asked whether cloudflare intended to convert that segment.
