I completely agree with your justified disappointment. Apparently, iTunes gift cards are no longer worth anything if you are a Family Share user. My kids are in elementary school and each has their own iPad. It really made sense to use Family Sharing. But since iTunes gift cards are not accepted as payment for purchases in a Family Share environment, I have $150+ in credits trapped in my account.
This is an exceptionally short-sighted policy, Apple, and not the first time I have been incredibly displeased with your draconian policies. I think the lesson here is for us to start planning our exit from the restrictive Apple ecosystem. I love the software, I like the hardware, but I abhor the oppressive policies. So consider us just another future former customer.
All our current iPhones are coming off contract this month and our iPads are started to get dated so it might be time to fully commit to the exodus. We've long since outgrown our iPods, we already replaced the Apple TVs with Google Chromecasts, our iMacs and MacBooks have a number of years left on them and aren't married to the iTunes store in the same way as mobile devices, and we have zero appetite for that ridiculous Apple Watch.
You lost a fanboy, Apple, and I've been with you for a great number of years and, as a technophile, have swayed many households to make the switch to Apple. Now I'll be helping people to switch away.