IIKings is bang on. It's strictly a lawyer thing and - yes - a tire thing. On the 300M, they put a limiter. On the 300M Special, they came with 18" Michelin Pilot Sports low profile - a high speed rated tire - and no limiter. DC took a lot of heat over this decision because you had to put very expensive winter tires on this car as it was undrivable on snow or ice with the summer Michelins (there were other major issues with the 300M Special - I'll outline in a future post).
When the C came to market, there really wasn't an appropriate all-season high speed tire. Hence the decision to program a limiter. It's my understanding that the SRT-8 will have performance tires and will not have a limiter. It will also not ship until after the winter.
It's my guess that C owners won't have to wait much longer before an aftermarket program will be available to remove the limiter.