Why does night seem less likely? I'd expect many of the sensors work equally well or better (no sun interference) at night, and there will be a lot fewer other cars to crash into/cause a traffic jam when the car shuts down and has to be recovered.
The fact that the speed limit is much lower I think backs up that Cruise's permit is actually less permissive, and is most likely for at-night only.
Waymo has had driver-free cars in Phoenix for over a year, so it would be strange for Cruise (who doesn't have a driver-free robo-taxi service already) to get a more permissive permit than Waymo.
