This is one of Amex best strategies.
A charge card that is paid late, no matter how much money you owe them all they charge is a late fee as apposed to Credit cards that charges interest and can be way more then the late fee
They offer this feature to ppl that have a good payment history as a "Courtesy", and then they rake in the interest
Never let them it apply it to your account!! even if they offer you 10k points (there is a reason why they want you to opt in)