You're not harming anyone or anything so there should be no law against it.
Actually you are. You cannot easily maintain control of your car when you are doing an emergency braking move or swerve if you are not held into your seat. That will increase the chances of other occupants in the car or other cars getting hurt.
Also, people who are badly hurt in accidents because they are too cool (even though nobody else really cares) or fat to wear a seatbelt are incredibly expensive to treat. This uses my tax money, increases the fuel price (through the RAF) and increases my medical aid contributions.
It also points to a state of mind which is part of the reason why driving in SA is the way it is.
I own and use a sports car - no, not a 'hot' hatch

, so I am not one of those crazy super safety advocates. The point is wearing a seatbelt is really such a tiny inconvenience.