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.
IT looks like they were held up magnetically, and therefore they were placed where they could be high enough not to be stolen, and on a magnetic metal object where they could be seen.