This law actually has a sad and dark origin.  After the civil war, many former slaves can not find work due to the decimated economy in the south.  As a result, there were alot of starving former slaves who would fight eachother over the work of cleaning horses so they can afford a meal.  Many of these fights turn bloody and some turned deadly.  As a result the law was passed that the white folks who need their horse cleaned are not allowed to openly offer to have someone clean their horse by calling for them.  And, the term they used to call for former slaves was “oh boy”