You're a cashier there aren't you?
This shouldn't come as a surprise to anyone who has worked in retail or for a company with a return/refund policy. There will always be people looking to game the system.
The company has the flexibility to change their policy. If they see it as a big enough issue they can change the policy to refund only the tendered amount, or conversely, start deducting a re-shelving fee for returned items. However, since retailers in turn get reimbursed for the coupon amounts, they probably don't much care.