But if there was technically nothing they could do to access the data, they could not be forced to do it. If I encrypt data on my machine and upload it to s3, A

But if there was technically nothing they could do to access the data, they could not be forced to do it. If I encrypt data on my machine and upload it to s3, Amazon cannot be forced to do anything other than provide the encrypted data to the government. They can't be compelled to give my key because they do not have it. It is very hard to operate e-mail this way, because the server cannot send your e-mail other servers in the encrypted form, unless you limit correspondence to users that are using your encryption and key-exchange system. Not all cloud services have this limitation. However I suspect that there will always be some metadata that the government will be able to request from the cloud provider, and this legal question will still be relevant.