Originally Posted by AeroGut

Explanation I have heard is that "proper" bicycle turn signalling in countries that drive on the right is done entirely with the left hand, so CPSC mandated rear braking for the hand still on the bars (ie right hand) because of a (misplaced) fear of the danger of stopping with only the front brake (the logic being that beginners have trouble modulating front brakes and stop too quickly and get thrown). In places like UK where they drive on the left, the reverse applies, and right hand is rear.
Cyclocross riders often switch the sides, depending on which side they typically dismount on for a bike-carry.