The trivality of the incident. We usually do not see people arrested for jaywalking or speeding or spitting on the sidewalk either.
Of course, all of this begs the question of whether throwing milk on someone ought to be a crime in the first place.
Chocolate milk throwing does seem a bit trite, IMHO. On the other hand, would you want to be one to make a list of the acceptable items that can be thrown and not thrown? How about a glass full of water thrown by an annoyed woman in a man's face in a bar? Eh, no big deal. How about a 5 gallon bucket of water dumped on an elderly lady who told some kids to get off her lawn? Where's the line with regard to quantity and "grossness" of the stuff thrown? Does the age, race and gender of the victim count?