In situations such as this one, I just remind myself how many times I've been reamed by airlines when it comes to last minute ticketing and times when they've refused to accept a valid coupon, etc.

It really does all even out. In this instance, you are taking advantage of the airline. If you are a FF, I'm sure there are/will be numerous instances where they take advantage of you.

In short, all's fair. They didn't collect it. Use it. You didn't ask for this and you didn't intentionally try to avoid handing the coupon over.