What are you talking about? It's not a membership. It's a physical item with a set price. Once it's paid for, whether monthly or up-front, it's paid for. Why on earth would you be charged for the extension period? The extension period is literally you getting to use the product that you paid them for.
Huh? It's not like a gym membership at all. It's like buying a car and paying for it monthly instead of all at once. Disney is extending the expiration of the pass, so why wouldn't you still make your payments until you've paid it off? Are you under the impression that having an AP is like being in some kind of a monthly club with membership dues?