It's a liability thing, pure and simple.  Vehicle manufactures have
figured out that a car they sell to the public MUST understeer in order for
them (the car maker) to avoid lawsuits in the US..So they spec. a tire
pressure that will cause understeer, even if a vehicle is capable of 100%
neutral handling.  Then, when "Joe-average driver" goes into a corner too
fast to have a hope of making it....The front end understeers and the car,
truck, van, whatever plows right off the outside of the corner and the
accident report will read something like "Driver drove off the road, going
too fast"..

  You may recall some oversteering vehicles from the past, such as the
Corvair or some of the Air cooled Porsches...Get into a corner too hot with
one of those, and the rear end came around ("Oversteer")..Those accident
reports probably read  "Lost control of car, spun off the road"...causing
the legal vultures to jump in and blame the car maker for building a car
that spun out...(no matter that that car was operated stupidly)

  So, we get almost all cars with built in understeer, or when they don't
have it built in, like perhaps a Vanagon (dunno, I am not brave enough to
take my Vanagon onto a road circuit and try it out at 4-wheel drift speeds)
then the manufactures specify a tire pressure that will cause it to
understeer...thus covering their a++es from lawsuits..

  Don Hanson