Prouty discuses this subject in the chapter 'Spiral Dives and Dutch Rolls' in his book "Even More Helicopter Aerodynamics'. The following is a partial excerpt from this chapter.
"The U.S. military allows the airplanes it purchases to have slightly unstable spiral-stability characteristics. The military is satisfied if a fighter does not double its bank angle in more than 12 seconds or a transport in more than 20 seconds.
The FAA requires helicopters that are certified for instrument flight rules (IFR) to not be subjected to spiral dives. You can see the result of this policy on the Bell 212. Before it could be certified for instrument flight, it had to be modified to decrease its directional stability by the addition of a vertical 'destabilizer' ahead of the rotor."