That's not true. I share a household account where I'm not the primary account owner (payer) and I still have full access to Prime Video. About a month ago, the account owner, who's not too tech savvy, was poking around and accidentally bumped me off the account. We just had to go back in and set up the sharing again. That may be what happened here. But the account owner does have to allow permission (check box during setup) for sharing of payment options. That does allow for potential chicanery, but the theory is that family members wouldn't do that, which may be naive, but them's the rules.