It depends on the country's health system.
In countries with publically funded health care available free at the point of access, wearing seabelts can save a lot of medical costs, allowing public funds to be spent better elsewhere.
Thus it is reasonable to make it law to wear seatbelts (or motorcycle helmets). Althouugh by not wearing a seatbelt they don't increase the chance of direct harm to others, they indirectly take resources from others who need it.
In countries where there is no real public health system and people typically pay for their own medical costs, the government has no incentive to protect the people who don't wear seatbelts, as their poor choice doesn't cost the government anything.
Unlike DUI, not wearing a seatbelt will not cause direct harm, nor will it cause indirect harm as the injured non-seatbelt wearer will be paying for their own costs (or their insurance company will).
I do think any civilised country should fine people up the wazoo for not having safely secured childen in vehicles; their parents might value the liberty of not wearhing a seatbelt more than their face, but it's not fair they make that choice for a child.
