32nd and what? There are plenty of bike racks surrounding Penn Station that you could have used. Some people can get real persnickety about their personal space, especially here in NYC, so bummer about the mean old ladies. Next time an "excuse me" might do the trick, though it might get you a huffy sigh.
As for locking to public signs, I'll lock to street signs often and have no problem (the street cleaning-type signs are the best for width), but it might indeed be technically not allowed to lock to public street signs. NYC's street signs tend to be a little close to the curb anyway, or in obnoxiously trafficked areas like the one you used, so I'll usually hike a block or go across the street to find a better place in such instances.