This is an old debate, some conspiracy theorists think amazon added it purely to get people talking online about kindle
hats off to you for actually time lapsing it lol, how creepy would it have been if on one frame screen changed to "look behind you"
ahem.. think its mainly just a common sense thing, like hmm, say if they left the book onscreen then powered down rest of system, alot of people might think kindle is brocken since when they hit next page button / type on keyb nothing will happen hehe. Similar but not to such an extent as that, if they just left the screen clear, alot of people might think kindle was actually dead
And ofcos it stops people from just being plain old nosey, picking up kindle and reading page your on
, if the screensaver has changed.. someone has been snooping :P