Heat pumps should be able to work down to about 20F outdoor coil temperature, and will still not build up a significant amount of frost or ice on the coils.
Manufactures are chicken, and set the thermostats around 40F to be safe with their warranties! And if there where to be ice collection on a outdoor coil, it might prevent the outdoor fan from running on a really cold day, thus prevent the electric heater from working properly.
(that comes into effect more with PTAC air conditioners like used in Hotels and I have 97 of them at my retirement center! I could only wish the PTAC would spend more time in the heat pump mode rather than the energy consuming electric heat mode. It takes 3 times as much electric to run a electric heater than a heat pump.
My guess is that manufactures try to change over to the furnace at around 40F temperature at the sensor inside the outdoor unit, that is near where the discharge air is located, so it can be cooler than the actual outside air temp, as the heat pump is discharging air that is about 15F cooler than the air going into it.
Fred.