Q: Why isn't store credit used for family purchases instead of billing to credit card?

Hello, Musket451. 

Thank you for visiting Apple Support Communities. 

Here is some information that explains how iTunes purchases are billed when using Family Sharing. 

How are Family Sharing purchases billed?

When you initiate a new purchase, it will be billed directly to the family organizer's account, unless you have gift or store credit. First, your store credit will be used to pay the partial or total bill. The remainder will bill to the family organizer. iTunes Store credit on your account isn't shared with other family members. So if the family organizer has a $50 credit on their account and another family member initiates a purchase, it won't deduct from the organizer's iTunes Store credit, but will bill the organizer's credit or debit card.

The family organizer will get the receipts for any of their family member's purchases.

How iTunes Store purchases are billed

http://support.apple.com/kb/HT5582

Cheers,

Jason H.