It was my understanding that it was a requirement. But if you didn't have it as original equipment you could make it switch operated. I may have read the requirement wrong because I knew I was going to put them on anyway.
Personally, I'd highly recommend doing it even if not required by code. Especially with tinted windows. It's a good safety feature AND it helps to communicate with other drivers.
I doubled up on the utility and made them rear facing work lights as well, operated by the switch.
On the down side... I leave mine on accidentally all the time while driving around.