Got the same thing for a neck massager. Those Chinese never sleep. I wouldn't think it's a scam, it's just simply a way to collect reviews.
1) They're collecting reviews to sell a completely different item with the reviews generated by the shoelace "giveaway". That is a scam.
2) Leaving a review in exchange for a GC without disclosing it in your review* is not only against Amazon's rules but it also goes against FTC laws. Doing that is being dishonest and as much as it is nice to get free stuff, it destroys your own ability to trust reviews the next time you want to buy something. It may not be a "scam" in the way people use the word, but it is most definitely dishonest.
3) Was your massager shipped by Amazon? Did you receive the note in the mail *after* you received the item, or was it a package insert?
*The only way to leave a review while disclosing you received it for free is through the Vine program. If you disclosed that you got a gift for the review it would b taken down. So there is no way to *honestly* leave a review on Amazon while receiving compensation from the seller.