Point taken, but I still think that the device should be protected against supplying it with a "malicious" input voltage, as there are cheap and robust components to do this. The NCP373 for example (http://www.onsemi.com/pub_link/Collateral/NCP373-D.PDF) protects against voltages between -30 to 30 V and is designed exactly with the "faulty USB cable" use case in mind, and the low-volume cost is just 50 cents (large-volume cost is probably much lower). So I think there really is no excuse for letting a faulty 5 $ USB cable destroy your 1000 - 2000 $ device, even if it supplies an input voltage of the wrong polarity.