I am a former US Army OH-58A/C guy. The US Army Kiowas had a Vne of 100 KIAS, doors off. Supposedly, speeds faster then that resulted in a form of control reversal - the aircraft response would not always follow a predictable pattern for a given cyclic input. One of the practical reasons for this limit was the back doors, with the original handle design, would blow open in flight. In cruise this was not a problem; however, landings were a different story. The reason for the slower 206 Vne is probably due to several factors - actual airspeeds flown during certification, etc. Hope this helps.