The furnace defaults to on to save the water pipes. The sprinkler defaults to off to conserve water as the system is potentially unmonitored and a burst pipe could cause issues.
A thermostat and controls are a necessary requirement for HVAC systems and defaulting anything to "run" if your control plane doesn't exist anymore is definitely not the safe option.
The other issue is that in almost all situations (like this one) what you think is a safe and sane default won't align with what other people think.
There should be defaults and they should be clearly defined, but I don't think it's always obvious to determine what they are.
While I agree with your overall point, this clause is irrelevant to/not supportive of it. The presence of a thermostat wasn't going to help you here either and there are vastly more furnaces with connected thermostats than disconnected to worry about.
CO detectors and alarms are needed to address this risk.
Freezing water pipes are bad, but a furnace running non-stop is going to exceed its duty cycle and pose a greater hazard.
Furnaces have multiple checks when they turned on, even on the dumbest furnaces. There are multiple safety mechanisms preventing it from getting too hot. CO leak - what thermostat will do for you here?
I had a friend in Australia who ran cattle on his farm. Failing open would waste water, but failing closed would mean dead cattle (and hundreds of thousands in losses). It depends on the application.