So the school has a no weapons policy. A #2 pencil makes for a more dangerous weapon than those little molded plastic toys. I certainly hope the school proxy servers block such dangerous sites as this!
http://www.instructables.com/id/Office-Weapons/
The school did have a choice. They had the choice to see that hat for what it really was, rather than try to rationalize some tiny pieces of plastic into dangerous items to be used to destroy, defeat, or injure an enemy.
Also, had some thinking educators been on site, they could have used the hat as a 'teachable moment' to create a dialog on the differences and possible relationships between military and patriotism instead of "You did it wrong, now go home!"
As I was typing this, I wondered how much more ridiculous this could get, so I googled the terms "expelled for making gun with hands" and found the following:
Yakima kindergartner expelled for making a gun with hands
http://www.kndu.com/global/story.asp?s=11979866
Personally, I find the little green 'army men' toys to be much more dangerous than a kindergartner pointing a finger. I have stepped on an army man or two in my time and it didn't feel very good. I have yet to be injured by a finger being pointed at me.