Good morning Tom,
I am not sure that was the primary reason at all but certainly a benefit. The truth is that DSM is safer for the simple fact that you do not have to worry about frequency conflicts. This has always been the biggest risk in RC is the possibility of someone turning on a transmitter while you are running your model. I don't believe anyone is being "forced" to do anything, it is just a requirement for NAMBA Sanctioned Races, like any other safety rule. When 72Mhz was banned for surface use for safety I am sure there was some grumbling, or the switch to FM. I think this is a positive evolution of our radio technology. JMHO.