To answer some questions here:

"Why doesn't the Nokia charge through USB"
- because that would be impractical. When you plug in USB, the two flash drives (internal + SD) are made available to the host computer and are no longer available to the Nokia. If you want to avoid using an AC adapter, get the nifty USB-to-charger cable. That allows the Nokia to charge from USB without mounting drives

"Why doesn't the Nokia run without a battery"
- because this would easily lead to unexpected power-loss and un-controlled shutdown. This carries a significant risk of corrupting the flash (internal+SD) file systems (possibly even the internal system drive). With a battery in place, a controlled shutdown can be initiated when the battery gets too low (but before the battery is completely drained). Note: Some networked attached hard drives have a similar provision in that they turn off write-caching (=> reduced peformance) unless they are connected to a UPS (detected via USB connection to UPS)

Martin