In the United States, bicycles are regulated as toys by the Consumer Product Safety Commission (rather than as vehicles by the Department of Transportation).
Requirements for bicycles being sold, new, at retail, are specified in Requirements for Bicycles 16 C.F.R. Part 1512.
Concerning the placement of hand brake levers, it says: Unless a customer specifies otherwise, the hand lever that operates the rear brake must be on the right handlebar. The lever that operates the front brake must be on the left handlebar. A lever that operates both brakes may be on either handlebar.
The purchaser may specify the other orientation.
This requirement only applies to the initial sales of new bicycles.
This orientation is the opposite of that specified by the Department of Transportation for motorscooters and motorcycles.
There may be some state or local requirement that augments this, but that's unlikely.