Some of the people who were threatened were locals who were federal employees, hence federal jurisdiction.
And again, if you are going to pull over six well-armed people who have committed crimes, you need more than a cruiser and a deputy.
Why these people of all the people in the world should be presumed immune to traffic stops is an unanswerable question.
They're not immune. If the cops stop you on the road, you stop and you cooperate.
That's how every normal person operates, unless they have some kind of agenda.
You yourself would never try to lead the police on a chase or reach for a weapon when a peace officer told you to put your hands on your head.
You would pull over and ask: "What's the problem, officer?" and if he said "I'm taking you in on a charge of x" you would say: "I'll come quietly, but I am not saying a word until you bring me my attorney."
I know which side you will be on when the civil war comes. Big government, nanny-state apologist.