This would be quite harsh, given that (a) the user does not control Amex reimbursements, (b) most of us are getting refunds for reasons that have nothing to do with the fact there was an Amex reimbursement (e.g., I found a better timed award flight), and (c) if they're going to do this, they need to do it in a way that allows us to reuse the credit.
If Amex claws anything back from me, I will definitely raise a stink about it.