That's assuming that it's a simple process to publish a huge list. Keep in mind that Safe Browsing is much more complex than an adblock list. They allow actions from site owners via Search Console, and track many types of threats (phishing sites, malicious downloads).
My guess would be there isn't a simple canonical list, and it's more of a heuristic evaluation where certain thresholds will trigger site warnings.
In this case an API would be more up-to-date, and less computationally expensive than serving large, always-changing lists. It was likely just the more logical choice. Though I'm assuming the new ads functionality uses the same Safe Browsing infrastructure.
>Sure they can say they don't want the offenders to know they are offending
Actually not the case at all. They send alerts to sites that are affected by Safe Browsing via Search Console.
