If from the guys who put them up, that is one thing, if it was from the advertising agency that is another whole different ball game.
It really must have upset the ad agency that for over two weeks their publicity stunt was not working in about 6 different cities. Maybe some idiot decided to get the ball rolling. Hard to image anyone that stupid, but ya never know.
I can actually imagine a panicky employee worried about his job, and he thinks "Hey, I'll just call 911 and tell them I saw a "suspicious-looking sign". After all, they are signs, and they obviously were "suspicious-looking" because the police couldn't tell they weren't bombs.
Would it be criminal? Maybe so, but would an average employee at a marketing agency see it as a crime to truthfully report that a device was hanging somewhere?