> Why doesn't Amazon say "hey, this is publicly available, you wanna fix it?"

They do. It's clearly warned about in the interface both at the time you make it so and with a big "PUBLIC" sticker label afterwards. What's more, I've received warning emails from AWS notifying of (intentionally) public buckets.

Public buckets for private data are a deliberate and wilful choice by lazy, reckless administrators.

I noticed this when I went into my console the other day. (Ironically, to set up a public bucket for something.) I don't think the UI was bad before but, now, you'd have to be pretty clueless to have a bucket public by accident.

You've always had to make it public deliberately. The new console UI makes setting a bucket to public a very deliberate effort.