These cards are just like using a rewards card at the grocery store -- they use you for data mining. The very information you provide them, which tells them where you live and what you buy -- and consequently, your general income, marital status, etc. that can be gained through OTHER databases -- is much more valuable than the $5 they give you. This is why a person may call this a scam. In the end, you're actually losing out from the deal, because Best Buy is making out like a bandit.
Of course, it's your perogative. In a very general sense, you are getting something for nothing. From the perspective of a person who does data mining on a daily basis, I tossed my card in the trash 5 minutes after I got it.
Now, if you REALLY want to get something for nothing, I suggest you find someone out of state through this board, and trade cards. Suddenly, you're getting the good end of the deal!