The iPad can charge from the USB/charging port, as can many Android phones, but this just isn't a supported feature on the Xoom. It's disappointing to find out that you need to carry more than one cable and can't use popular emergency battery systems with your Xoom, but the Xoom is hardly the first piece of portable electronics that can't charge by USB. Your netbook can't charge that way, either.
To charge your Xoom, you need to either use the charging cable that comes with your device or purchase a cradle accessory designed to work with Xoom. Do not plug in any charger that was not specifically designed for charging a Xoom. If you find that your Xoom is not charging as expected, make sure the charging cable is securely plugged in to the device, and then try restarting your Xoom.