My take exactly.. Wherever I am, not just Thailand, when i get into a car, the belt goes on. (Here in UK, the police do enforce the law rigorously.)
I REALLY don't like motorcycles and try to avoid them at all costs - my choice.
And that's the crux of it... I CHOOSE to wear a belt in a car (front or back seat) and it's become second nature. BUT I really don't think it should be law to prevent me from harming myself, unlike the anti drink-driving laws which set out to prevent me from harming other people.
However if I want to be so stupid, enter me in the Darwin awards!