I know from your absolute conviction on this (coupled with LOTS of experience with people who have absolute conviction about stuff) that your own conviction is preventing you from seeing valid uses for this, and is potentially keeping you from seeing the 100% of the landscape you're professing about.
There are real uses for AWS Buckets that are public and cost you money. Distributing files, acting as a webhost, anything that you’d use dropbox with link sharing for.
Yes, it sucks if someone randomly decides to download files from you all day. You should probably set your budget to alert and attempt to blacklist them when it happens. That’s rare, though, and aside from a few cases of actual malice, the convenience is worth the cost.