Ask HN: Why are "save password" checkboxes ever defaulted to "checked"?

I've always been puzzled by the thought process behind having "save password" checkboxes checked by default. Facebook, eBay, Yahoo, and even my bank website are some examples of major websites exhibiting this (Gmail used to do this, too). I'm thinking that there must be some logical reason behind wanting the default behavior to be that the first line of security is completely dropped for subsequent visits. Anyone have any insights here?