Why Are In App Purchases Not Included in Family Sharing?

So I’ve purchased the app Heads Up on our iPhones. I have Family Sharing I am the organizer, for my family of 3. I’ve purchased 99 cents each, several “decks” as in app purchases. I couldn’t get my other family members to get the decks “free” since I paid for them and thought since we have Family Sharing they should. So I contacted Heads Up they said “Apple won’t allow in app purchases to be thru Family Sharing”. Is this true? If it is that is ridiculous why wouldn’t this be considered part of Family Sharing? Thank you