PeterH:
On the other hand, I suppose there might be some cost to monitoring the pressure, and the designer might have designed it to only do that when it immersed. I suppose the resistance measurement would tell it that, too.
If nothing else it might be an energy saving feature. The OP mentioned that dive computers turn on automatically below a depth of 3 ft (and presumably they turn off when above), so this type of automation also could be eliminating one thing a SCUBA diver will have to have on their metaphorical checklist before a dive.