Here is a great shot of the Main Street ramp entrance. So, this is all Ruby had to do is walk down that ramp. And it's what he said he did. Nobody had to open any doors for him.
So, Officer Sam Pierce was parked at the top, sitting in a police car. And there was another officer standing outside the car talking to him. Look how narrow it is. Ruby walked right by them. He recognized Pierce. They had to see him. Weren't they there to keep people out? Why else would they be there? So, why didn't they keep Ruby out? Because Ruby was welcome.