Some of the countries that don't require you to wear a seatbelt have roads in the main cities that are so overcrowded their really isn't any need for them as your not going to damage yourself that badly at all.
Also Some of the poorer countries where not everyone can afford a car, have to cram a family on a motorbike or a ute full of school kids in the back to get to school. If seatbelts were made illegal alot of these kids would be walking hours to get to school and would be very impractible