Because, uploading before pressing the "save this publicly" button is counter-intuitive to most people. Much like if texts were sent before I pressed the send button -- until i press that button there is the option to change my mind, edit what i say, etc. This is the mental model most people will have, way more so when presented with a "publish" button (akin to a send mms button).
> Much like if texts were sent before I pressed the send button -- until i press that button there is the option to change my mind, edit what i say, et
Except that analogy doesn't fit. Other users cannot see, nor do you see on your account, the uploaded-but-not-explicitly-published photo. It only appears publicly when you choose for it to be.
A text message that was sent before you pressed the send button would have to be held at an intermediate (such as BBM servers or iMessage servers), and of course makes little sense due to the size of a text vs. the size of a photo.
I understand the technical aspects of it. There is still no guarantee that they delete the photo. They say they do, and I don't seriously doubt them, but the possibility exists that they keep them around for whatever reason. Further, there is a real possibility that an attacker could find a way into the Instagram systems or hijack the communication streams between users and the servers, and then you have a leak. It is a potential privacy concern that is caused by the "upload everything and throw away some of them" algorithm.
