You keep referring to the Terry Frisk law, it is not a law only a court case decision. There are many other factors that allow an officer to search you other than the Terry Decision.
You also mention that they did not have probable cause to stop you, probable cause is needed to make a warrant-less arrest, not stop. In order to stop they only need reasonable suspicion.
You thought your actions were not suspicious, and by your thought process they probably were not. But you need to look at what the officers knew, and when they knew it, in order to determine if you were suspicious to them.
Young people waiting in a parking lot, when the business is open, and they are not going into the business is point one of suspicion. The time of day, the crime rate of the area, the number of calls in the past on that lot, are all just a few more things to consider. When you start adding them up, you can get a suspicious car stop. And while SC law does not permit traffic citations on private property without posting, it does not prohibit law enforcement from conducting a suspicious auto stop.
No, I do not believe your rights were violated. I do think you did the right thing and cooperated. If more people cooperated with suspicious car stops, less people would find themselves in custody.