Content Restriction applied to store and game library

If you buy disc it should still show up in your digital library, ergo your 10 "xbox family" members should still have access to it. Same restrictions apply, only one person playing it at a time. Sell back to a store, game is removed from digital library.
This is where things are murky, the unique disc id has to be de-tethered from your GamerTag.

well that error doesnt just apply to free games

it applies to any content with region restrictions

like AAA titles like cod or halo or fable or assasins creed

Hello BartoszFigura1,

Thank you for bringing this issue to our attention. That is a great question. The downloadable content must be purchased on the account. The license for the downloadable content is then applied to your account. The game can be downloaded from the Game Pass
library, a disc or the Xbox store as a digital purchase. The console or computer will allow access to this content as long as the downloadable content and game are installed, and the console or computer is signed in to the correct account. Be sure to let us
know the results!

It's actually due to restrictions in the 360 store that don't apply on the One store. In order for child accounts to purchase backwards compatible games, they have to be purchased on a 360 console.

If you're on a child account, you would need to purchase the 360 games from a 360 console due to child restrictions. The 360 store is set up differently than the one on the One/website, so any restriction settings you have on the One will only apply to the
One store.