> It is surprising that they didn't make it compatible with the S3 API .... The lack of scalable front-end load balancing is shown by the fact that they require users to first make an API call to get an upload URL

And... you answered your own question. :-) We reduce our operating costs by not having as many load balancers in the datacenter and pushing off the responsibility to the API. It all comes from our traditional backup product where we wrote all the software on both sides so we could save money this way.

With that said, we are actively considering offering an S3 compatible API for a slightly higher cost (basically what it would cost us to deploy the larger load balancing tech).