[thenewnewguy]

Cloudflare's primary raison d'être is messing with the response they serve to users - to perform caching, to inject CAPTCHAs when they detect a DDoS attack, etc.

If you don't trust Cloudflare to not abuse this to inject ads, stop using Cloudflare.

[ozim]

They can mess with response to make redirects/checks before my content is served, but my content is mine they should only cache it and that is it.

If they serve ads on their captcha or some redirect I'd say well it is not nice - but for me fundamental difference is messing with my content even in a good faith - send me a notification an email or stop serving my content if it is active malware but don't change it.