Here's my hypothesis:
From the images that I've seen of these, it looks like the signs were designed to light up at night. (There's a pic over at http://www.flickr.com/photos/vanderlin/358742603/ of one of these in Philadelphia in operation.) At night, they light up, it's a cartoon image, no one gets alarmed.
In daylight, they're not lit. Someone wanders by in the daylight, sees a weird circuit board, call the cops suspecting the worst. Other people recall seeing the same thing as well and call the cops. And then we have a city-wide panic.
The Boston bomb LEOS know what is what.
They knew as soon as they saw anything that size that the amount of damage it could to to a bridge would be cosmetic.
If it was actually an IED.