I think the federal requirement is that the manufacturers have to provide them with the car.. To get a ticket for it there would have to be a local state/county/city law against it.
Local cops can't enforce federal laws, and I've never seen a federal highway patrol before.. hehehe
I guess if you live somewhere where you get inspections every year you would have to be concerned, but otherwise, go for it if you don't want the airbag.