[wstuartcl]

The assumption is that the correct passcode will remove the usb shutoff. If you fail to enter the passcode in the required amount of times, you get a wipe. Many of the law enforcement ways to access these devices rely on the USB port being active to root the phone or reset it in a way that allows faster/more passcode attempts (or by simply letting it sit on a shelf for months or years until a known exploit allows access via usb.