Why would AWS provide a feature that makes additional transaction and storage charges (as well as subsequent reads to see which is the correct version) irrelevant?
S3 (and most AWS services) are extremely price elastic; i.e., the lower you make them cost, the more people use it (a la electricity.) That's why they've done stuff like drop from 50ms billing to 1ms billing, etc.
They could still offer it as a client library feature, just tell the users what kind of r/w amplification and guarantees they can expect, and it's something they could optimize later or not.
