Seems they don’t want helicopters or possibly other flying cameras over the police activity in Ferguson, MO…“to provide a safe environment for law enforcement activities.”
http://time.com/3105035/ferguson-faa-no-fly-zone/

“It’s just for a no fly zone because we have multiple helicopters maneuvering in the area and we were having some problems with news aircrafts flying around there,” the dispatcher, who would only identify himself by his first name, Chris

http://thinkprogress.org/justice/201…uson-missouri/

Others feel it’s also in place to keep drones from being used as well.

http://gigaom.com/2014/08/12/faa-imp…ful-right-now/