Because form Android's perspective "in use" means that you are touching the screen. The OS doesn't care if the screen is changing or not.
With TouchDRO (if you're using it as intended), you should be hands-free most of the time. I.e. set up the workspace/layout, turn on auto sub-datum selection, and go.
To be clear, the flag to keep screen on works ONLY when TouchDRO is in the foreground. It is per app. You'll notice that some other apps do this too. Many higher-end tablets have a feature that uses the camera to recognize your face, and as long as the camera registers you staring at the screen, the tablet won't go to sleep.
As I mentioned, one could set screen timeout to something very long, but that means that you would know that this is a thing, and how to do it. Many don't, and screen turning off in the middle of an operation can be a major inconvenience. TouchDRO will preserve the position and any of it's state, but if the screen goes blank when I'm single-point tapping a 10 TPI thread into a blind hole, blank screen would at least mean a scraped part.
But going back to my earlier comment, when you are done, tap the power button. I don't leave my tablet running when not in use, and don't find a button click to be too burdensome.
Regarding carried position - how does that work with other DROs? With glass/magnetic scales, the scale would need to be powered up. I understand how one could save last known position, but unless a DRO has a backup power source, any movement will desynchronize the scales. I haven't seen this on any of the import DRO's I've touched. Do the "big guy" DRO's come with a built in battery?
Regards
Yuriy