How was it okay for a cop to watch a car without reason

So I was at a Sheetz with my boyfriend and we saw a cop pull in cockeyed at the gas pumps he actually got out of his car and walked around the one we drove there. He then pulled out and parked across the street to watch the car. So my boyfriend walked out beside the car and watched the cop pull away. However, the cop then turned around and parked at a Valero and watched for us again. My boyfriend who was driving at the time was following all traffic laws to a T didn’t even get fully turned before the cop pulled out and started pursuing us. Yes, we were wrong for running and having a truck licence plate on the car but can a police officer really do that without having a reason to pull us over at first?

Traffic Lawyer Richard Simon’s Answer:
So I have a way of looking at these types of questions. Can I watch your car without any reason in a public area and could you do the same to another person. The answer is yes. The plain view is a common police practice. If the general public can do it, generally so can the police.