Got kicked out for being under 18

Me and my friend both 17M decided to go to Dollar Tree because we wanted some snacks. It's a Friday night, and I didn't eat. We walk in,look at the food, then my friend randomly gets talked to by someone who works there and says we have to have a parent with us if we're under 18 because of the increase in theft. This really frustrated me, and it felt like he was accusing us, even though we were just standing there looking at the food. There was a sign on the door saying no children allowed without an adult. I just want to know if this is allowed because the paper was signed by the "store manager."

I'm not going into huge detail about the situation because it's a lot to type out, but this doesnt feel like its legal in anyway idk.....