Then what's the excuse for Japan being so much higher? They've had a lot of wireless access for quite a while, and web browsing on their phones for much longer. If that's taken into account, they should be significantly lower.
It's not an issue of wireless penetration, it's about maturity. Obviously the first rollout in the USA is going to be slower than a mature infrastructure like Japan's.
Surely when you roll out new wireless, you should roll out the fastest stable technology? Wouldn't Japan be held back to some extent by wireless infrastructure that hasn't yet been upgraded?