This is the equivilancy of pointng a cardboard tube in broad daylight at someone.
The guerilla marketers did this without a hitch in over a dozen cities. These things look nothing like bombs.
How many bomb scares do you call in when you see somone drop a can of coke on the sidewalk?
You're assuming that these were labeled bombs by the people who called into the authorities. Have you heard the recordings of these calls? Was it one call? Or were there multiple calls describing strange objects at various locations across the city? Were you there? Did you observe these items hanging from under the bridges? In the subway stations? At the hospital? Maybe they don't look like bombs after they've been taken down and photographed by the press, but did you see them in situ? What expertise or knowledge do you possess to assume what these look like to other folks or what was said in the phone calls?