[dsmid]

eschwartz is right, what is often (mistakenly) interpreted as "turned off" is just a sleep state with blank screen. The device is not actually turned off.

When you turn the device off with /sbin/poweroff, it has zero power consumption and the battery will last months (every battery loses some power even when not used at all, albeit Li-ion batteries have a slow auto-discharge rate).
When Kindle is in sleep mode (=suspend to RAM), some circuits are still on power and it can also wake up to perform regular checks, thus the battery will not last as long as when turned off completely.
That's why they ship Kindles powered off.