No, I believe you are incorrect. The dongle must be manually turned on and manually turned off.
That's the problem, it doesn't turn itself on and off with external power, so if you shut down your Spyder and walk away, the dongle remains on using it's internal battery. The dongle battery will eventually run down and the unit will not function until it's charged up again.
Some people have solved this problem by changing the power source to an "always on" connection, instead of a "switched" connection. That way, the dongle is powered by the Spyder battery, which has a much higher capacity than the dongle internal battery, but over a long time, say for instance winter, this will also drain the vehicle battery, unless you have it on a battery tender.