I think it's because people tend to associate the cabs with the slow speeds involved in the city. I typically don't buckle up in Manhattan, except on the FDR or the West Side Highway, or if we're going above 30 mph.

For those unfamiliar with the NJ turnpike, where the accident happened, the speed limit is 65 mph and people drive like maniacs. Definitely a place where you want your seatbelt one.