On my Pandora, due to bad internal WiFi, I use an Edimax EW-7811Un USB WiFi adapter, which has a status-LED, flickering according to the network activity.
After disabling USB-host through the settings menu, the WiFi functionality is gone, but the device's LED is still on. So some current must still flow into the USB device.
The LED now has a permanent light, no flickering! This lack of "device intelligence" indicates that the device was indeed turned off, just current flowing directly to the LED circuit unaltered).
I can repeat the aforementioned steps many times, the behavior is the same.
As soon as I then first physically unplug and reinsert the device, the LED remains off eventually.
From then on, every further issuing of "Disable USB-host" through the settings menu turns the device LED completely off.
Smells like a kernel bug to me! As it requires one real un/re-plugging, from then on works ok.