They do it because they can.
From The New York Times:
It could be the cutbacks to the city’s transportation system, or a general decline in urban civility. Perhaps people are just in a collective bad mood.
What else could explain why New Yorkers — notoriously hardened to the slings and arrows of everyday life here — are spitting on bus drivers?
Of all the assaults that prompted a bus operator to take paid leave in 2009, a third of them, 51 in total, “involved a spat upon,” according to statistics the Metropolitan Transportation Authority released on Monday.
No weapon was involved in these episodes. “Strictly spitting,” said Charles Seaton, a New York City Transit spokesman.
And the encounters, while distressing, appeared to take a surprisingly severe toll: the 51 drivers who went on paid leave after a spitting incident took, on average, 64 days off work — the equivalent of three months with pay. One driver, who was not identified by the authority, spent 191 days on paid leave.