I always figured that ford did it to reduce emmisions. Think about it going WOT with 0psi of boost in an older PSD, such as a 7.3, you would get some black smoke. This is why when your running down the road with 20psi of boost the throttle response is better, cause the computer realizes you have boost, so it will give it all the fuel it will inject stock. This is why when you power brake it to say 30psi, you have hardly any lag, the throttle response is great. This has always been my thinking.