American allies are usually perfectly fine to defend themselves. I know one of the reasons for the ANZUS treaty was America was the only country that had the power to successfully attack Australia.
If America did pull out of these countries there goes favorable negotiating conditions for trade and such, there goes safe sea lanes, there goes a safe oil supply, up will go the proliferation of weapons and with it tension and possibly war. I really think America gains a lot more selling weapons and pushing their laws on allies then what would be gained from a solely continental defence strategy.