Dollar Tree

I have visited this particular store twice and I've noticed that they like to close the store before closing hours. The sign says that the Dollar tree closes at 8pm, but instead they close at 7:45pm. The very first time I visited this store, they closed at 7:50pm. This time they closed at 7:45pm. There were people still shopping inside but the employees locked the doors to that new customers couldn' come inside. I've visited other dollar trees before and this has never happened to me. However, this store doesn't mind closing a little early. I guess it's okay for them to do that?