Not sure where you heard that, but until the citizens of the US become less sue happy and more wiling to be responsible for their own actions (be it via technology or otherwise), Toyota wouldn't dare risk putting something that could easily equal lawsuit. My guess is they also figure we're not "smart" enough to figure it out, or more accurately, that we're probably too lazy to read the manual to figure out how to use it correctly :-P (or something along those lines)
My take on the auto parking is that Japanese are more polite; Americans are more impatient: Drivers in Japan will, but U.S. drivers will not, wait for the 5 minutes it takes the computer to parallel park the car. Picture New York drivers backed up behind a car that's taking 5 minutes to parallel park on a busy street!