Don't underestimate the power of defaults. When you send someone a photo through most systems the recipient has to go out of their way to delete it, so they win

Don't underestimate the power of defaults. When you send someone a photo through most systems the recipient has to go out of their way to delete it, so they wind up keeping (and re-sharing) things they didn't even intend to. If you want to circumvent Snapchat's retention policy you have to actually care enough at the moment to do so, which is a much bigger barrier than any technical one.