Pershing Square's mysterious zoning

NYC can be very tight with commercial zoning. excessive fines are passed out for tables or signs outside that go an inch over the allowed space. but every summer Pershing Square Café, located on the very busy corner of 42nd St. and Park Ave., is allowed to SHUT DOWN and take over a block of Park Avenue and fill it with outside seating for their café. in doing this they block an important south bound Park Ave turn off of one of the busiest streets in the city! this baffles us to no end (adding to the insult, the place is often empty.) does anyone know the story behind this restaurant being allowed to take over a city street?

UPDATE: Venus Williams will be appearing in Pershing Square TODAY at 12:00! playing tennis for a good cause!