I've successfully prefiltered the lists to include only one, volunteers, however, when I update the contact, it unsubscribes the contact from the other lists. How does one only show the list to add and not unsubscribe from the other lists. Do you just hide the other check boxes?
Using the Sample Form, it will update the specific contact to be in only the lists that they actually check. So if the list the contact was originally in is not displayed on the form, it will remove them from those lists.
The best way to do what you are looking for would be to do a Search by email address for the specific contact, which will retreive the contact's ID. You can then do a GET in the Contact Collection URI with the Contacts ID, this will give you a list of the contact lists that the email address is assigned to. You could then add these to an area in your form, allowing customers to check them on or off, or do not display them, but you would be able to include them in the update.
The sample forms are more meant to show examples as to how to do various abilities with our API's; however, I do see how this could cause confusion, and I will submit this feedback to our engineering team for review.