Or you could.. you know... recommend an appropriate client-side encryption tool so they can then store the archive/backup data on the storage provider of their choice...
The advantage of having client-side encryption built into tarsnap is that it encrypts only after data deduplication and compression.
Obviously there could be a tarsnap option to stream the data to be uploaded through an encryption program of your choice, but doing it just as you suggest would nerf a few of tarsnap's prime advantages.
Tarsnap is a combination client-side application and remote service.
I am suggesting instead to use/recommend one of the existing client-side tools that work similar to the tarsnap client does, but don't lock the user into a single service provider.
By using a client-side tool that just generates archives (and isn't tied to a single storage service provider), you can store them anywhere - AWS, iCloud, Google Drive, Rsync.net, a rented VPS, a friends computer, an external hard drive, all of the above. You name it.
I understood what you said, I just didn't know that there were tools in existence that are as good as or better than tarsnap at the archiving part which allow you to specify the storage location.
Edit: I used 'specify the storage location' very loosely. I.e., I realise it could mean simply piping the archive data to yet another program in the shell.
I've never used it, but I've seen people on HN recommend Attic. It dedups and encrypts. https://attic-backup.org
Personally, I use git-annex, which isn't exactly a backup tool but a general distributed file manager which can, among other things, automatically make encrypted copies of the files to various places (SSH servers, S3, Google Drive, etc).
Both do dedup and encryption, Attic can also store the data remotely via SSH (either with or without installation on the remote end) and Obnam can handle remote storage to an SFTP server.
It's an almost completely transparent user-space filesystem. Basically you store your files in a given folder, and it automatically stores a parallel encrypted copy in a different folder.