It seems to me impossible to distinguish between transferred miles and earned miles in an active account.

If:
I have 100,00 miles in my account, of which 50,000 are transferred from a FB Family member.
I then spend 60,000 on Award flights.
I am left with 40,000 miles in my account.
How is anyone going to identify where those miles originated?

I can't see how transferred miles can be treated any differently from earned miles. They surely have to be subject to the same rules as all other miles?