The following is a quote from "Department of Health And Mental Hygiene Board of Health - Notice of Adoption of an Amendment to Article 161 of the New York City Health Code" (http://www.nyc.gov/html/doh/downloads/pdf/public/notice-adoption-hc-art161.pdf):
"In addition, rabies remains endemic in New York City and the numbers of cases of rabies in wildlife appears to be increasing: from six positive cases in 2003, to 14 in 2004, doubling to 28 cases in 2005. Wild animals, many subject to rabies, reside in City parks, and also enter New York City from suburban habitats, seeking open spaces in the City, including those in parks. While rabies remains endemic in the City, all dogs must be vaccinated against rabies, in accordance with Health Code §§11.65 and 11.66. However, when dogs are off the leash in park areas and facilities and not under their owners’ constant observation, the dogs are at greater risk of exposure to rabid animals than leashed dogs."
Makes perfect sense to allow dogs to run free in the parks, especially when the Department of Health and Mental Hygiene acknowleges that only about 5 percent of New York City's dog population is properly licensed.