Speaking as a citizen of a country with universal health care, there is a very good argument for governments enforcing seat belt use here. In the event of a crash, a seat belt significantly reduces the risk of serious injury, and hence the cost to the tax payer to provide medical care. This is true even in the case of privately insured individuals, as most of the major trauma centres are government run, and the insurance companies pay only part of the cost
Even in a country such as the US, without free universal health care, there is still a strong economic argument for enforcing seatbelt use. If as a result of a serious crash, you suffer long term injury or death, then there will be a decrease in your productivity (especially in the latter case). Therefore, a decrease in production, and reduced tax levels, hurting both the individual, and the capacity of the government to provide services. While I'm not familiar with the US social security system, I suspect an accident resulting in permanent disability would also provide added costs in that fashion.
We gave you an atomic bomb, what do you want, mermaids? -- I. I. Rabi to the Atomic Energy Commission
