Why doesn't 's3cmd sync' support PGP / GPG encryption for files?
What the s3cmd sync command does is:
Walk the filesystem to generate a list of local files
Retrieve a list of remote files uploaded to Amazon S3
Compare these two lists to find which local files need to be uploaded and which remote files should be deleted
The information about remote files we get from Amazon S3 is limited to names, sizes and md5 of the stored files. If the stored file is GPG encrypted we only get size and md5 of the encrypted file, not the original one and therefore we can't compare the local and remote lists against each other.