John Deere is to be commended, one supposes, for being much more concerned about blackhat terrorists than e.g. Siemens with all their internet-accessible SCADA installations, but neither ISIS nor Putin care about killing a couple of farmers in western Iowa. Thread parent was about automobiles, and while maybe professionally-operated vehicles like semi trucks or buses would be better with which to start using ammonia on a large scale, vehicles are unlike applicator implements in that they aren't actually designed to release ammonia into the environment. Conceivably a car could be hacked to lock the brakes at high speed, but there's just no mechanism to pour out its fuel tank.

How do you know that the tank won't rupture in an accident? Such engineering is not cheap.

Ammonia is less flammable and explosive than gasoline, which is commonly found in currently-engineered and currently-existing fuel tanks. They are both much less flammable and explosive than hydrogen, which is the other fuel under discussion in this thread. Ammonia tanks would be pressurized, which is different than gasoline, but lots of tanks are pressurized.