The mandatory provision is written in to the underlying CC&R documents for the resort. This is why they can't easily take it away. Voluntary resorts are missing the language in their CC&Rs documents. As long as you buy resale at one of the listed mandatory resort that is available in the stickies, those StarOptions will transfer and can be mixed and mingled for reservations with other StarOptions you have.
On the Resale Information Sheet (estoppel), Vistana actually mentions "Voluntary Resorts", however for mandatory they reference them as "Club Resorts". So if you see Club Resorts checked on the Resale Information Sheet, you are good to go if you are buying a mandatory property. However, errors happen so it is always best to know the list of mandatory resorts and phases.