Quite true, anyone can park in the casino garage. The only reason to show a PAC card is to get free parking. You simply swipe it at the exit to the garage to avoid having to pay.

BTW, make sure that you end up in a room that offers a view of both Falls. The rooms that only have a view of the Canadian Falls look directly at the side of the adjacent Marriott. The rooms with views of both the American and Canadian Falls have an extremely memorable view.