For many people, the thing being substituted for an alternative is not "transportation into Manhattan", but more broadly "engaging in commerce in Manhattan"
What business or shopping trip into Manhattan is small enough that 9 dollars is a significant hurdle or increase in cost? It's there to disincentivize taking a car for no reason when you can use transit while being small enough to absorb if you have a reason to actually use a car.
What business or shopping trip into Manhattan is small enough that 9 dollars is a significant hurdle or increase in cost? It's there to disincentivize taking a car for no reason when you can use transit while being small enough to absorb if you have a reason to actually use a car.