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A casino is private property and so is the parking lot. If you were removed from the casino, and/or told not to return, they can keep you off their property. Since the parking lot is also their property, they can prevent you from returning to their parking lot. They would have expected you to take your vehicle when you left or have someone else pick up the vehicle for you. So, yes, although unfair, their conduct is legal because they have the right to exclude people from their property.
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