South Carolina bicycle law mirrors many states with regard to bicyclists' rights and responsibilities as shared users of the road.
Firstly, Section 56-5-3420 states cyclists must be granted all rights and are subject to all duties applicable to a driver, except where the law expressly states otherwise. Further, Section 56-5-3430(A) states that "as practical," cyclists shall ride as near to the right side of the road as possible.
It's that "as practical" section that many cyclists base their decision to ride in the middle of the lane. The reason is because when traffic is allowed to pass a cyclist who is to the right of the road, there is greater potential for injury to the cyclist. This is why absent a bicycle lane, many cyclists opt to ride in the middle of the road, with motor vehicles having the option to pass either using the left lane or crossing the center line when there is no oncoming traffic.