Seatbelts Save Lives

While I was driving, my cousin told my sister that they need to wear rear seatbelts. I asked her wether it has been enforced or not. She said it's going to be on the 1st of January, but they need to get used to it early. Don't you think the enforcement of this rule (or law?) has a flaw in it? 

Think about it, there are three seatbelts at the back of every normal sedan. What if there are more than three passengers at the back? The police responded to this question with this. 'Well we cannot do anything about that, but the driver can be compounded for overloading. 

What about the big family which the parents cannot afford big cars? What about the concept of car-pooling? It is a good thing that they are enforcing the law, but still, to me they did not do any research before making a statement.