Flight this week diverted due to weather. I had a meeting to get to and figured out that I could make it by driving. Door opens and gate personnel indicates that no one is allowed off until Captain approves. I get my items and go to the front of the plane and he physically stands in the door and tells me to back away as no one can get off. I did not want to cause any problems so I backed away and waited the 5 min. for them to sort it out. Is what the UA gate agent did legal? I believe when the door is closed, it is the Captains call, but what about when the door is open?