Dude, that poor USB connection can only provide so much juice!! It'll provide some nominal power, but not a lot; 500 mA per port if I remember. Heck, most USB webcams draw the allotted amount; a scanner does quite a bit more.
Edit: Primascan website says "In the United States, the Primascan 2400u scanner is available at Circuit City." Heh, I find that amusing, considering Staples is the one providing us with this great deal.