(Of course, I'm assuming we're not dealing with a zero-day in the USB stack or filesystem drivers. But that probably is something that the Secret Service should be on top of, as well.)
In essence, modern OS's give "autorun" privilege to keyboards and mice. That's the HID in this discussion -- Human Interface Device.
What does autorun have to do with a mouse or keyboard device? The problem with USB is that you don't know if it is a "mass storage device" or any of the other kinds of devices that can start interfacing with your computer.
If it is an (automated) keyboard device (HID), it will immediately start "typing" which means it can open a terminal window and start executing things.