IIRC, Motorola implemented this feature early into the ASTRO25 radios because users scanning both conventional and trunked personalities in the same scanlist would be constantly affiliating and unaffiliating from the system every time a channel grant was broadcast and the scan delay timer expired, and it was overloading the system. P25 can only output 40 single TSBKs per second, and even less on a multi-block system, so having the datastream saturated with unit registration requests and the subscribers walking over each other is a recipe for disaster.
They found out quickly that only affiliating on PTT when it's receiving from an unaffiliated state on a scan list is the better way to go.
I'm unsure if the original ASTRO 9600 firmware (R06.72) is capable of true NAS.