I would imagine that if a boat or ship were to hit the footing of the bridge it would cause considerably more damage than an R-22. Must seagoing craft remain 500 feet from the bridge as well?
Risk is rather relative. Has a scientific risk assessment been conducted to assign the risk of the helicopter hitting the bridge? Can the authorities cite examples of helicopters unintentionally hitting bridges? Is there a documented case of a helicopter bringing down a bridge by collision? I saw a video of a CH-47 fall onto a bridge in Korea while placing an Olympic Torch. The bridge is still standing.
There are many helicopter operations around the world where helicopters routinely operate near or around structures at distances of less than 500 feet. As an example a tourist helicopter company in San Pedro, California made every take-off and landing under a bridge for years without incident.
I would be more concerned about the risk to the helicopter from dangling cables or litter thrown from the bridge and for that reason would consider the risk high.