Many societies have negative views toward the habit of spitting in public. Spitting on one's face is an act of insult and potentially assault.  In 1872, the Illinois Supreme Court upheld a jury judgment of punitive damages for a person who was spat upon in his face in a crowded courtroom. Alcorn v. Mitchell, 63 Ill. 553 (1872).  The court held that "[i]t is customary to instruct juries that they may give vindictive damages where there are circumstances of malice, willfulness, wantonness, outrage and indignity attending the wrong complained of. The act in question was wholly made up of such qualities. It was one of pure malignity, done for the mere purpose of insult and indignity."  Thus, courts regulated spitting as an assault and battery.

In the 1890s, scientists found out that spitting was not only offensive but was also related to tuberculosis rates.  This disturbing discovery led to the first anti-spitting law, a New York City ordinance from 1896:

Spitting upon the floors of public buildings and of railroad-cars and of ferry-boats is hereby forbidden, and officers in charge, or control of all such buildings, cars and boats shall keep posted permanently in each public building and in each railroad-car and in each ferry-boat a sufficient number of notices forbidding spitting upon the floors, and janitors of buildings, conductors of cars and employees upon ferry-boats shall call the attention of all violators of this ordinance to such notices.

 
Legislatures identified spitting in public as a costly externality that should be regulated and New York City was only the first jurisdiction to act. Other states and cities followed.  Over time, tuberculosis almost vanished in the United States and anti-spitting laws generally disappeared.  States that still have them, do not enforce their anti-spitting laws. 

Spitting, however, has remained a socially symbolic act of offense. In 2010, New York City faces a new problem: spitting upon  workers of the City's transportation system is classified as an assault under the drivers' contract.  The New York Times reports  that Passsngers' habits to spit on drivers result in significant paid leaves.  Thus, although less costly than tuberculosis, spitting has remained socially costly although perhaps for different reasons.

Another way to look at the social cost of spitting in New York City is of course to focus on the drivers as an interest group that inflates the social costs of spitting in modern times.

See:
Michael M. Grynbaum, When Passengers Spit, Bus Drivers Take Months Off, New York Times, May 25, 2010, Page A1.