Why do you feel OK taking advantage of a buy-one-get-one-free deal? These stores aren't stupid. They know people do this. They do a cost-benefit analysis on their return policies and decide that they're OK with this gaming because it is compensated in other areas. Their salespeople can tell prospective customers, "Hey, you're not committing to anything. If you don't 100% want to keep it, you can just bring it back with our awesome return policy." They do this knowing that most people won't even if they're not completely happy. The flip side is that we actually get to take them up on the offer.
On the other hand, I don't personally do this because I'd rather just work a little more and properly rent or buy. Seems like less work in the long run.