The reason buying an open box laptop and returning it two weeks later hurts retail companies is because of lost opportunity cost. This is especially true for products that have a short life cycle, such as computers. Those two weeks you have the laptop are two weeks the store could have been trying to sell that laptop. When it gets the laptop returned, it is now two weeks closer to the next cycle of laptops, better models. Once the new models arrive, the 'old' one will be even less attractive of a sale and the store will either have to reduce it's price further or write it off as a loss.
Also, you're putting 'wear and tear' into any product you use for two weeks. That depreciates the life of that product. For instance, the hard drive may lose some of it's potential life due to the bumps it takes while in your possession. Maybe you get some food in the keys or the lcd gets a small scratch that the store doesn't catch when you return it.
If you want to get very detailed... the store spends money on the paper and ink that prints the receipt that you received when you bought the laptop. Further ink and paper is used to print the return receipt. There is also electricity used to run the registers that you used. Don't forget about the small amount of packaging tape needed to re-seal the box again after you return it.
In the end, the store DOES spend/lose money for those two weeks you have the laptop. They gain nothing... and you gained the use of a 'new' laptop for two weeks... time a 'legitimate' customer could have been using the laptop had it been in the store available for purchase instead of with you.
