Not sure either but the general consensus is that it is a disgusting thing to do.
Quite why it may be may be seen as worse than snapping a person leg in two , by some, I simply do not know.
As an aside and possibly may explain why but I have a vague recollection of a policeman in Swindon getting killed by a virus in the phlegm of a city fan back in the 70's, who spat on him.
A quick google search brings up nothing so maybe this may be an urban lie.