Because they said so. And you'll stop asking questions, if you know what's good for you.
/
Sounds like the Military Police that I have run into. Why not let our local police have the same powers over us that the military exercise over our soldiers.
Especially troubling is the "presumption of innocence". That one gets in the way all the time. Let the accused prove that they are innocent. Saves a lot of resources and simplifies police training.