Does the shirt have a badge on it? If so, that's more likely to be seen as impersonating a police officer. http://www.ebay.com/itm/NYPD-T-Shirt...-/110936877520 Although "officially licensed" by the NYPD it may noit be "officially approved" by your local PD or prosecutor.
Are the shirts "raid" style with POLICE stenciled across the front and back? Again, it may rise to the level of impersonating a police officer.
Are the shirts like hundreds of other NYPD/NYFD "tribute" shirts? THis http://www.ebay.com/itm/Navy-NYPD-T-...item6fcd211486 is less likely to get you in trouble. Even less so if you wore this http://www.ebay.com/itm/The-Amazing-...-/170877669421 - although you might suffer public humiliation.