But how is an open, read-only S3 bucket worse than a read-only HTTPS site hosting exactly the same data?

The only thing I can see is that it is much easier to make it writeable by accident (for HTTPS web site or API, you need quite some implementation effort).

Only to gullible, clueless types.

Full blown production SPAs are served straight from public access S3 buckets. The only hard requirement is that the S3 bucket enforces read-only access through HTTPS. That's it.

Let's flip it the other way around and make it a thought experiment: what requirement do you think you're fulfilling by enforcing any sort of access restriction?

When you feel compelled to shit on a design trait, the very least you should do is spend a couple of minutes thinking about what problem it solves and what are the constraints.