What is the process for a TFR based on law enforcement activity?
TFRs around "volatile situations" on the ground aren't a daily occurrence, but they do happen fairly frequently.  

The concerns are usually twofold: Events on the ground posing a hazard to aircraft (people shining lasers at aircraft, firing guns at them, etc.), and aircraft posing hazards to each other (News 2, News 4, News 11, and Police helicopters all wanting to cover the same area at the same time and smacking into each other).

There are procedures in effect for agencies to request TFRs, and for the FAA to review them. These are covered in detail in FAA JO 7210.3Y, Chapter 19. Particularly relevant to this situation is 19-2-5 (e): "TFRs issued for law enforcement activities require approval from the ATO Director of System Operations Security (or designee)." - a relatively high level of approval intended to safeguard against the use of TFRs to, for example, keep the media away.



In the case of Ferguson the regulatory basis for the TFR is cited as FAR 91.137(a)(1) (to protect persons and property on the surface or in the air from a hazard associated with an incident on the surface), so presumably their major concern is laser illumination or weapons fire from the ground, and the potential for such incidents to cause a surprised pilot to crash into a building or crowd of people.