Keep in mind there are no laws in the US regarding airbag compliance for bumpers. All the Federal "energy absorbing bumper" regulations surround limiting body damage to your vehicle in low speed impacts (to reduce repair costs) and, for bumpers in general, protecting your lights and radiator.
Here's a place to start, if you want it from the horse's mouth, as it were.
http://www.nhtsa.dot.gov/Cars/Problems/studies/Bumper/Index.html
2) What is the purpose of bumpers?
The car bumper is designed to prevent or reduce physical damage to the front and rear ends of passenger motor vehicles in low-speed collisions. Automobile bumpers are not typically designed to be structural components that would significantly contribute to vehicle crashworthiness or occupant protection during front or rear collisions. It is not a safety feature intended to prevent or mitigate injury severity to occupants in the passenger cars. Bumpers are designed to protect the hood, trunk, grille, fuel, exhaust and cooling system as well as safety related equipment such as parking lights, headlamps and taillights in low speed collisions.
