I'm english and we always had the front brake on the rightside.
Here in france it is on the left side.
It is to do with making turns into traffic so that you can signal and have your hand on the rear brake.
It is considered safer to slow down with the back brake when signalling with your arm.

So in the US if you are turning left and need to wait for a gap in the traffic you should brake on the rear brake with your right hand.

Well that's the principle anyway.