Actually, that rule may be the one constant for snakes! There actually aren't any herbivorous or even omnivorous snakes in the world known to science (and I think it is highly unlikely that a herbivorous snake is yet to be discovered). Plant matter has been found in the droppings of snakes, but lab studies have concluded that this was by accident and snakes did not readily eat plants for the sake of eating plants (cottonmouths occasionally ate seaweed for no other reason than it smelled like fish, but they simply aren't capable of the digestion of vegetation).
This article by herpetologists from British Columbia delves a little further into the matter.
http://snakesarelong.blogspot.com/2012/10/why-are-there-no-herbivorous-snakes.html