Why is law enforcement acting like an occupying army?

They seem to think their job is to contain people, not to help them. Some quotes from The Interdictor:

National Guard drove over the bridge above them, and tossed out supplies over the side crashing down to the ground below. Much of the supplies were destroyed from the drop. Many people tried to catch the supplies to protect them before they hit the ground. Some offered to walk all the way around up the bridge and bring the supplies down, but any attempt to approach the police or national guard resulted in weapons being aimed at them.

and

Any attempt to flag down police results in being told to get away at gunpoint. Hour after hour they watch buses pass by filled with people from other areas. Tensions are very high, and there has been at least one murder and several fights. 8 or 9 dead people have been stored in a freezer in the area, and 2 of these dead people are kids.

The people are so desperate that they're doing anything they can think of to impress the authorities enough to bring some buses. These things include standing in single file lines with the eldery in front, women and children next; sweeping up the area and cleaning the windows and anything else that would show the people are not barbarians.

I suppose I should answer my own question: Because they are being shot at.

I asked what civil rights the citizens have and the US Marshalls looked at me like I just fell off the turnip truck and chuckled. I asked if citizens can have guns for protection and he said if someone thinks he needs a gun, he should have already evacuated. He also said they are setting the city on fire.

and

If you're watching the feed, it's incredible. Hard to believe the fire department is still viable. God bless them. I'm no expert on conflagrations, but I don't think they're gonna win this one. Hopefully they can contain it.

It takes a spectacular kind of asshole to set a fire in this environment.