Then toss in the transients. Ambient air temperature changes change tire pressure. Tires heat up under high speed driving, changing tire pressure. Pressure is relative, it'll read one thing at sea level, another when you drive up into the mountains. Beach driving or other soft-surface (mud) often requires you to lower tire pressure to increase contact patch size. Tire and vehicle manufacturers recommendations are based on average loading, light or heavy vehicle loading may require adjustments for optimal performance.
So, after all that, some big-government bureaucrat is going to try to tell me, and Joe down at the service station what the right tire pressure is? And just how well calibrated is Joe's gauge? Hey, I know, lets create more regulation and bureaucracy related to the gauges, more un-paid work for Joe...yeah, that'll help the economy.
If he used Craftsman Tools, then he would know for sure.
My Cousin Vinny http://www.youtube.com/watch?v=O-NsYBrTp9U