Why would business owners have cameras pointed at the highway if they wanted to catch people stealing from their scrap pile? This doesn't make much sense. Surely their cameras were for identifying people in their lots, not driving down the street. If that was the case, they would pay thousands in equipment and tapes just to record the road. i can see LE getting tapes though just in case a suspicious vehicle happened to turn around there or drive through their lot. But, the cameras being trained on the road...doesn't make sense.
Also, I can't see throwing a bucket off the bridge to be any big feat whatsoever. It's all in either just dropping it or giving it a good swing. Not saying this can be done from a vehicle, but at that time of morning, not much traffic around that area and, even if someone saw it and reported it...(wait...someone DID report someone throwing something off the bridge)...it either is another piece of information that LE has now or it wasn't checked out and now will be.