I think Patrick is being far too gracious in his "great software, bad business" dichotomy. The data deletion policy is all software, and is ridiculously, laughably bad.
I wasn't aware of this "feature" before, and I don't currently have any need for Tarsnap's service, but I don't think there is any way I would choose to use that service or any other software product from the same guy/team. Maybe the underlying cryptography is rock-solid, but that's completely irrelevant if my backups are going to be deleted while I'm on vacation.