It usually feels to me that if you don't want someone to take up the offer you need to have tighter returns policies. I see why companies do it, for the vast majority of people buying the product it's like an added bonus if something goes astray. This opens the system to gaming by the small minority of customers but still the benefits to the companies sales far outweigh the cost. If everyone was doing it I'm sure the companies would be a lot less generous with their returns.
Reminds me of a story a friend who works at EB Games (some campany as gamestop in america). Every week or 2 a lady would come in and exchange a game she purchased for her kids for another one, usually just falling inside the exchange period. Essentially she was using EB as a free game rental store. Eventually they cut her off but not before she had well and truly gotten value for money on whatever she had originally paid.