Note: https://github.com/bup/bup does that too (though it does not encrypt), and http://liw.fi/obnam/ does too (and it does encrypt).
What tarsnap gives you that obnam doesn't is (a) managed cloud storage, (b) tarsnap's history and reputation, and (c) Colin's personal reputation. That's a lot, and it costs money above the S3 storage costs (which you could point obnam at).
Note: https://github.com/bup/bup does that too (though it does not encrypt), and http://liw.fi/obnam/ does too (and it does encrypt).
What tarsnap gives you that obnam doesn't is (a) managed cloud storage, (b) tarsnap's history and reputation, and (c) Colin's personal reputation. That's a lot, and it costs money above the S3 storage costs (which you could point obnam at).