Typically an act of frustrating desperation, a cyclist has turned hailing a New York City cab into a congratulation. He's probably trying to get people to think about cycling instead of taxis, but really he's just scaring commuters and tourists.
Strictly speaking, striking another person uninvited is assault. Making that stick in court would be pretty hard, but still. People are minding their own business, trying to snag a cab, and this guy comes along and hits them. What if he struck a person suffering from rheumatoid arthritis? The pain would be terrible, and he'd be blithely on his way, assaulting more people.
He's an artsy-fartsy type who evidently thinks his artistic vision is important enough to go out and interrupt other people's days, even if it is only for a moment. Some are amused, some nonplussed, some frightened, some annoyed. Is the amusement of some worth the startlement of others? I don't think so. That, and artsy-fartsy types just bug me. Get a job, pansy. A REAL job.
One umbrella through the spokes makes up for lots of random minor assaults. New Yorkers as a rule put up with a lot of shit, but beyond the tipping point retribution is harsh and swift. Remember that when you're spread-eagled on the ground with your teeth artistically arrayed around your face.