What you have seen is the supplies design.
In a single camera setup. You did not have to send an On to its X10 address to start the video.
In a multi-camera setup. They where designed to be is a group of four. Same House Code; Units 1-4,5-8,9-12 and 13-16. Turn one in the group On the other three in the group automatically turned Off. Without the need to send an Off to switch off the camera that was On.
I will look and see if I can find any schematics.
If you want them to default to Off. I believe how they act is in the firmware for the controller chip and you will still have the group of four membership.
Some camera models with Instant On supplies had two outputs. On a 3.5mm stereo type plug. One was On all the time and the other was switched. I would think a switching transistor to switch the DC output.