"The driver is free to leave the lot without parking if he does not want to agree to these implied terms"

This is ridiculous statement, especially in an urban environment where parking lots fill up early and there may not be a reasonable alternative. Also, it's likely many lots are under the same ownership so they will all require arbitration, so leaving without parking is pointless. Either way, you have no choice; you can't stick your car in your pocket and carry it around. It's beyond belief that a sign can override a constitutional right to a trial.