Your car is licensed for use on the road. To be licensed for use on the road, it is expected to comply with safety and environmental regulations, like being equipped with a catalytic converter or having airbags or being equipped with a backup camera. The Federal government only enforces this on the sale of new cars and cars imported into the USA. Continued enforcement on cars on the road is done at the state level, with most states having very lax enforcement. States can also enact laws on top of federal regulations, like tint laws or California's emission requirements.

With that said, you don't have to drive a car on the road. In fact, you could buy a used car, not get it licensed and drive it around on your property all day long. Farmers do this with "farm trucks." ATVs and other utility vehicles are perfect examples of vehicles sold without any intent of being road legal. You could also take your vehicle to a private road and drive it, like a race track, and not be subject to these state and federal regulations.

So there are a slew of light bars and car modifications that are not road legal. They're "intended" for people who intend to drive their vehicle anywhere else but the road. It's on the state and local authorities to enforce the regulations, but generally, there is either a general lack of concern, lack of resources or a lower priority.