I live in a Normal Country. But I can put myself in other shoes.
Think about it, in the USA the large Majority of people have Guns in their Homes, so its not unlikely that if the Police gets a Call that there is someone who Shot his dad, and is holding the rest of the Family Hostage might be true.
I know its hard to understand for people living in Normal countries why one wouldn´t follow the Instructions you get from a Police Officer, like "Hand where I can see them". But you know, if the odds that you have a weapon are dangerous are so High, its just stupid to not show the Police that you are no threat.
I wont defend "Trigger Happy Maniacs", I defend people who according to the Report did everything they could. They shouted to him to Raise his hands, and walk slowly towards them. (Away from the Suspected Hostages, and towards the Police so they might be able to Talk) He did the Opposite, he lowered his hands to where he could Pull out a gun. (Which is quite Possible that he might have one).
Honestly, if I were locked up in a House that is drenched in Gasoline, with a Lunatic who just shot my Dad, I wouldnt want the Police to take the chance that the call might be a Joke.
Even in "Normal Countries" like you said, does the Police assume the worst case Scenario, the only difference is: The Usual Worst Case Scenario is way more mundane than in the USA.