Nothing wrong with an unmarked car. Standard police practice. Ask your own police department; if they've got more than four cars they probably have an unmarked vehicle, and if they don't they probably have the policemen's personal vehicles equipped for that use when necessary. There are too many legitimate applications for silent approach, stake-out, et cetera ad nauseam.
IF they are conducting unreasonable surveillance, that's an issue. But the cab is not evidence thereof, and not even especially suggestive thereof.. Especially in a police department the size of NYC's, which can afford to (and must) maintain an active detective department and vice squad in addition to constables on patrol.
If you wanna freak about police abuse, great -- but freaking about this vehicle just makes you look out of touch with the reality of big-city police work.
Heck, if anything, I find this sorta poetically appropriate. A lot of NYC's police cars, when retired from service, have traditionally been refurbished into taxi cabs. This one just got the new paint job while still on the beat. Beats the usual "make it boring" unmarked car colors.