Or their AI chatbot decides you smell funny and bans you forever with 0 chance of appealing to a human
Right thats the much more likely issue! Some Google-like reaction in the future where using service X (chat/mail/whatever) from Amazon results in being banned across an entire account which also happens to be the account you wrote all your data to AWS Glacier with for 30 years... oops!
I think the most realistic concern here is amazon scanning your backups. I totally get it. A lot of the reason for retaining total control over the storage medium is that what is being stored isn't strictly legal.
You should locally encrypt 100% of the information you store on someone else's system with strong symmetric encryption. In fact, I think Amazon's legal team would quietly prefer that you do this.
Dumping a bunch of bare files named things like:
into S3 buckets is probably a bad idea.