Apple does not explain their reasoning for the payment system but I can see it as being a great way to prevent misuse in sharing things with people who are not at a 'family' level of trust and familiarity.
From https://support.apple.com/HT212253
"If you're the family organizer and you don't want to pay for family members' purchases and subscriptions, you can turn off Purchase Sharing [ https://support.apple.com/108774 ]. When Purchase Sharing is turned off, everyone must use their own payment method. When Purchase Sharing is turned off, family members can share subscriptions, but they can't share individual purchases like apps or books." ...or movies
If you don't want things charged to your payment method, have others add gift cards to their account so that in most cases their personal balance will be used first.