I believe the iTouch 2G and later require that the USB socket it is plugged in to either establishes communication with the device (something a powered hub on its own won't do) or alternatively sets the two USB data lines to a fixed recognised standard voltage that lets the device know it is connected to a USB socket designed only for charging.
If the USB data lines are not connected to anything, then the device concludes it cannot communicate with it, nor is it connected to something it is safe to charge from (despite the +5V and ground lines being powered correctly), and therefore it refuses to charge from it. That made a lot of older USB chargers/hubs etc incompatible, as previously it would charge from any power source which provided the ground and +5V lines, but now it is essential that the USB data lines are set correctly as well, and to save money many older chargers didn't connect those lines as per the USB charger standards (to save costs, albeit literally just one or two pence in total)