I think you must be quite surprised at the responses you're getting to your offer of a free gift. But some things are serious enough that them being free is a serious red flag.
In other words, I've you had presented this as a commercial offering in stead of as a hobby and just for fun you would have given a better first impression, one that proved that you had thought through the problem before embarking on solving it.
HN has been a pretty strong critic of the way tarsnap is being run (specifically from a marketing point of view), the collective view is not that it is too expensive but rather that it is not yet expensive enough and that it needs to appeal more to non-techies.
You've solved none of the problems that tarsnap already solved and then on top of that you have added a whole pile of new ones. So I think that even though I applaud your generosity and your general approach that this is not going to go very far. Using 'tar' means you're targeting developers, most other people will have never even heard of tar other than that black sticky stuff. Developers know the value of their data better than anybody else does (or at least, they should) and they will evaluate a backup service based on that value.