Can you imagine what would happen if you went crawling around under a bus in New York City with a camera?? Oh, My God, call Homeland Security!!
Simply taking photos of the outside of a bus can get you into trouble...not because of terrorist paranoia but simply because of the people (and especially children) who might be in the shot. I heard a piece on the radio just recently where a bus-spotter was talking about the regular grief he gets nowadays whilst going about his hobby.
(I'm not sure whether or not 'bus spotting' as a hobby has much of a following in the States; I'm taking about odd people who find interest in recording bus numbers, just as they do with trains and aircraft. Nothing at all like the perfectly normal hobby of taking photographs of steering linkages on quad-axle Scanias).