Garmin apparently has a different opinion about what a device should do when plugged in to a power supply.
Some Garmin devices turn themselves on automatically when they detect an external power supply.
Some of their devices it can be controlled in the settings what happens when external power comes on or goes off.
It seems with the 66SR that setting cannot be controlled as Garmin in their wisdom didn't include it.
From the Manual:
When you connect the device to a power source, the device turns on.
Perhaps to charge it, plug it in and leave it plugged in, then once it's finished starting itself up shut it down again, it should continue to charge and will charge faster than if it's fully powered on at the same time.
I have a Montana 610 that powers itself up when it detects external power, so I leave it plugged in and once it's finished starting itself I just shut it down again and it continues to charge.