The obligation for a termination notice, time to cure any violations of terms, etc is something that would be in a contract you agree to. A service provider is able to have terms that allow them to cut you off instantly if they decide to put that in their terms - apparently, CloudFlare has terms like that.
Would I want my ISP or CDN to cut me off without notice? No. Do we know they actually cut KF off without notice? Also no - it's possible this is after they gave KF's operator time to respond. But also, I wouldn't operate a site like KiwiFarms, so I'm not terribly concerned about my CDN cutting me off for the same reason. If it became more of a widespread pattern with CF I would understand their customers getting nervous.