Can officers stop you for just registration violation?

State of Georgia. My husband and I were driving through a small town when we passed a police officer sitting by the train tracks. We crossed over the tracks and went to the traffic light, turned right and proceeded to the gas station and got gas. The officer pulled over and sat across the street from us. We pulled out of the station went through the light and headed out of town when he pulled us over and informed us our registration was suspended and we had no insurance. We didn't know, however, at no time did he say we had done anything to make him run our tag. Is this legal?