What's the point of a "flipped" opamp power supply (going from -9V to GND)?
A possible reason is that some of the IC's, like that IG021610 part, seem to be designed for a negative supply (the power pins are actually labeled GND and VEE). So the other ones that can be used either way are just adapted to that supply. Their signal inputs and outputs are capactively coupled, and so the absolute voltage at which those chips operate doesn't matter.