I have to admit that I haven't thought about this much, and I only get coupons from the papers. We get two copies of the local paper at the library where I work, and the Director said that I could have the coupons from them as a perk for being the one who works every weekend (what a deal!)
I have no experience with the coupon selling/trading services, but when it is a matter of family or friends passing them around, what would be the difference (except a lot of hassle) in them giving you the coupon, or buying the item itself with the coupon, then selling the item to you for the price they paid for it? Would that be illegal? Or unethical?
I wonder if that wording is just there to give the companies a recourse if there begins to be some enormous abuse of the coupons that really did make the company incur losses. Also, there is the matter that most of them say "Do not double" but some stores double and triple them anyway. Is that just coupon-ese telling the store to give out as much money as the want, but they will only be reimbursed the face value of the coupon?
I agree with what others have said -- that this is a marketing ploy, and if it weren't profitable or benefitted the company in some way in the world as it is - which includes the coupon trading that the companies know exists -- it would be stopped.
Would it be any more unethical to take full advantage of all the coupons you can get than to go into a store and purchase only their loss leaders? Isn't the coupon in a sense a "sale by invitation only?"
Anyway, a lot of words to say I don't know much about it.