All payments/billing goes through organiser?!

I've recently setup family sharing, as well as Purchase Sharing. I understood purchase sharing was anyone who's purchased an app, would give the other family members the ability to download for free.

I have a total of 5 family members, 2 adults and 3 children. Both myself and my wife are setup as Adults/Guardians and im the orgniser, and we all have purchase sharing enabled.

My wife has just setup a small subscription on her own device, however ive just received an email to say its going to be taken from my preferred payment method (the email states my last 4 digits of my card, in the email to me).

Is this normal behaviour, for her purchases and subscriptions to be paid via the organiser? I would have expected the other "adult" to pay via their own normal payment method.

Thanks