I have done a little research on this, and it seems that the rev6 boards are missing a diode that keeps power from the USB from backfeeding into the ESC's to power them. Because of this, the method above is the only way to calibrate a rev6 board and unless you want to solder in a diode to stop the voltage from getting to the PDB, it's just one of the quirks of the rev6 board. Only other option is to get a rev5 board if you can find one.
But yes, motors spinning on USB power is very bad. I highly agree.