The wire-in resistor is for current limiting — believe it or not, we’ve had customers who mistakenly plugged in 24VAC to the sensor port, without a current limiting resistor, it will burn out the microcontroller.
There is no external pull-up resistor because the microcontroller has built-in pull-up resistor that can be turned on in firmware, which is exactly what the firmware does during initialization:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/OpenSprinkler.cpp#L840