How does the seatbelt law work?

I got pulled over and received a ticket this morning for not wearing a seatbelt. As soon as I saw the blinking lights, I put the seatbelt on. So, technically it was on when I was stopped. Can they do that? I know its illegal to ride/drive without a seatbelt, but I didn’t think they could give you a ticket for that unless you were pulled over for another violation as well. I live in DeKalb county Georgia if that helps…