Imaginary conversation on how this condition was created in IE:
MS Engineer: "Zoinks! IE's cookie/password store on disk is totally open to password sniffing by rogue applications when IE is running! We need to re-write the cookie/password store immediately!"
MS PM: "Sorry we don't have time for that as we need to get the foundations for the HTML5/JS hype machine out the door in time for Build. We need to burry the failure that is Silverlight/WPF as quickly as possible. How about you wipe the cookie/password store off disk when IE is opened and use an in-memory copy."
MS Engineer: "But if the computer looses power the user will loose all of their stored password..."
MS PM: "Well we shouldn't be storing passwords for them anyway. That's a bad practice so we're doing them a favor. Let's also not forget that you can be replaced..."