It isn't necessarily a scam. The cards are from aftermarket extended warranty companies. You get them when the regular factory warranty is about to expire (not sure why you got one already, not sure how they get their info), whether or not you have an extended warranty already. The companies involved can often be legit (however legit you see warranty companies as being).
Personally, I steer clear of aftermarket warranties; you usually need to front the cash for repairs and then fight them for reimbursement, and that is more trouble than it is worth. The factory warranty is seamless and provides more protections than any aftermarket one (usually covering rental cars and more). Aftermarket warranties are usually just prepaid repairs, and aren't a given. You'd be better off banking the cash for future needs. If you don't use it (or even if something huge breaks and you don't want to bother fixing it), you can always use it as a down payment on a replacement.