They panic, then act irrationally. Add to it that a large percentage of drivers are barely capable of driving when all the stars align perfectly. Throw a wrench in the system and all bets are off. I get your point on how police are supposed to be highly trained drivers. I don't know what to tell you, but hitting the brakes, shifting to neutral or holding the on/off button for 3s would have saved him.
The article also mentioned that many other manufacturer's on/off buttons are set up so that multiple rapid panic-like presses will turn off the car, not so with toyota, so it is plausible that many people may not be aware of how to turn off their toyota while it is in motion. The supposed reason for requiring a hold is that accidentally hitting the button and it instantly turning the car off would result in dangerous scenarios for most drivers. I personally dislike the whole on/off button in the first place. Additionally, turning the car off should be the last option as you will lose power steering and brake assist.