Personally, I've always thought it should be against the law to leave a ball with nothing attached but as far as I know, there's no such law.
I believe all passenger cars are required by law to have 5MPH bumpers - a receiver and ball defeats that so is preventing the safety equipment from doing it's job. Similarly, I don't believe 'brush guards' (
https://www.google.ca/search?q=brush+guard&client=firefox-a&hs=B0d&source=lnms&tbm=isch&sa=X&ei=y5IcUsy-M6W02wXiuIF4&ved=0CAkQ_AUoAQ&biw=1680&bih=932) are illegal (although I think they are in the UK).
As far as needing a flag, in Ontario, you only need it when the 'load' overhangs the vehicle by 5' (1.5m) so this wouldn't be an issue for any trailer hitch & receiver (although one thing most people probably don't know is that by law, at night, a flag is not sufficient and you need a red light (I suspect that other State / Provinces have similar laws)
