Domestic use of military forces is not an uncommon thing around the world.

[JHB]

6. Because if all you have is a hammer everything begins to look like a nail

A millitary force is a hammer of the biggest sort. Its purpose is to destroy the capacity of an opposing force to act. There are several hundred years of tactics and strategies to do so, but that is the basic function.

Police forces are charged with enforcing laws and keeping the peace. The training, skills, and equipment for that role are different than for military forces.

It's generally considered a Bad Thing when your own citizens become "the enemy" in the eyes of those charged with police duties. There's a tendency to get heavy-handed in their enforcement, acting more like occupiers than police forces. This happens when those duties fall to military or over-militarized police forces. That was recognized in this country over a century ago in the Posse Comitatus Act of 1878.

Yes, there are plenty of places where those duties are mixed. Some of those places don't have the best track record of protecting the rights of their citizens. Others are just small, so they have to rely on basically the same pool of manpower for both functions.

Ironically, as I write this there is a helicopter I can't see hovering nearby. I think the visibility is mostly a line-of-sight matter, and its probably a police helicopter over an accident or something.