They are, obviously, signs. But the features, such as exposed battery packs, exposed wiring and method of placement, play on the fact that people are looking for just such things in bombs. This was intentional.
Instead of parroting the official Boston police description, why don't you look at the video linked at post 36 of the construction and placement of the silly little lighted signs. They look nothing like bombs, and don't have any of the features you list. They are (I say are, since they seem to still be up in other cities where the police and general populace are not idiots) lightweight adhesive-back flat panels with a figure like a 1980's video-game character flipping the bird rendered in LED's.
This is blatant, inexcusable stupidity on the part of the authorities on the order of giving a kindergartener a three week suspension from school for having one charm on her charm bracelet shaped like a revolver, because the school has a zero-tolerance policy for weapons.
Did you think they are adhesive-backed? The way they went up, I assumed they were magnetically attached, which is why they put them on bridge abutments and other metal structures.