Everybody has been watching too many movies. That's why these devices were effective advertising. They were designed to play on the public's perception of "BOMB".
You keep writing things like that. HAVE YOU LOOKED AT THE VIDEO AND SEEN THEM FOR YOURSELF? (Sorry to shout, but you show evidence of not having done so, since you keep asserting that they looked like bombs.)
Comments on the video do suggest a hypothesis, though: someone complained about people putting up LED's and the handwriting of the police officer who took the call was bad, and the next person who got the message thought it said IED's.
Speaking as someone who has made more than a few electronic devices, I seriously doubt that the thought of bombs even entered the builder(s) mind. The placement of the signs was probably chosen for visibility rather than for some crazy notion that it would make these obviously non-bombs look like bombs. If I were having a yard sale (or something) I'd want to place my signs in heavily trafficked areas and the magnets are a great idea for making a somewhat heavy item easy to hang. Of course this requires ferrous metal next to heavily trafficked areas--hey, that sounds like bridges and overpasses to me.