How come FBM sellers are required to fulfill Free Replacement orders when the original item isn't scanned returned?

For example, when it comes to Refunds, Amazon will only give the customer a refund after the USPS (or carrier) first scans the item in their possession. Only then do we see a refund given to the customer.

But when it comes to someone who says an item arrived damaged, so they open a return and they are allowed to get a free replacement (coming across as a new $0.00 order) which we must immediately fulfill. But this is BEFORE the customer gives the return to the carrier.

Why this is a problem: the opportunity for fraud is high. A customer can claim an item is damaged, file a return request, then request a free replacement, and keep both items (original and replacement).

How come Amazon doesn't release a free replacement order after a first scan of the original, and how can sellers protect themselves from abuse by dishonest customers who wish to keep both orders?

(NOTE: What we end up having to do is try to keep track of these replacement orders, and then contact Amazon is the original was never returned. It's more of a headache for busy stores, when in reality, I feel that the free replacement order should not be submitted to sellers UNTIL a first scan is performed by the carrier. That's what Amazon does for returns, so why is it different for replacements?