I have a different theory: In Germany and Italy, coaster-braked bikes were the norm. On those, you have one brake lever only. So you put it on the right. If you add a second lever (for a rear brake), you put it on the left.
In France, coaster brakes never were populer. So they have the inverse set-up - probably for no good reason, except they may have started with rear brakes only! So the rear goes on the right. Don't know about the U.S... Probably adapted the French model.
For riding, it really doesn't matter. No good brake requires more strength than a left hand can provide, and anyhow, the variation in hand strength from rider to rider is greater than between hands of the same rider.
The funniest thing I've ever heard is that cyclocross might have something to do with it - you want to feather the rear brake when you have only one hand on the bars. If you do that, you probably won't stay upright!
What do you think?
Jan Heine, Seattle