One possible reason for the different standards between ST and LT or passenger tires is that ST's can only be used legally on a trailer and people don't ride in trailers.
You cannot, and should not, put STs on a truck or car.
The reverse does not hold true however. There is no law that I am aware of the prohibits the use of an LT tire on a trailer or car. Could be wrong on that but I have never heard of one.
Anyway, I think this is probably the main reason why ST's are not a choice for a passenger vehicle and would account for the different test standards.
Barney
