They're inserting 5 billion events per second (for some reason), and AWS throttles everything (the following link mentions a limit of 3,500/second):
https://aws.amazon.com/about-aws/whats-new/2018/07/amazon-s3...
Also, if they're planning to do alerting, S3 would not help with that.
PUTs are 1 cent per 2000: