The bucket's start out __private__.
The UI when making a bucket public comes with a big warning.
Then a big warning label is attached next to the bucket itself.
It does show I think the scale of AWS that the userbase is so large that this isn't clear. This tool is nice because it means that a perhaps more experienced admin can lock down an entire account a bit more so newer devs looking to shortcut things with world public aren't as able to (which will slow them down for sure).