Connecting USB devices wake Mac running Mountain Lion only temporarily
Siracusa's review mentions this behavior, and it's been evolving since Snow Leopard:


  DarkWake has actually been around since Snow Leopard, where it was used to periodically send out a network tickler to ensure that a sleeping Mac doesn't lose its DHCP lease. In Lion, plugging in or unplugging a USB device could briefly put a Mac into DarkWake to reconfigure the peripheral bus before going back to sleep. Mountain Lion does all of the above while also carving out some time to get real work done while the master is away.


DarkWake is the state the system is in when Power Nap is running.  In Lion, it would enter DarkWake to deal with processing the USB connection, then it would go back to sleep.  In Mountain Lion, when a USB device is connected, the Mac enters DarkWake to deal with processing the USB connection and also uses this state to do the various Power Nap tasks.