I think people have been reporting if they trade in a 360 system or something they get the $30 Credit on an Xbonx One controller. Trying to find out for sure what triggers it.
Edit: I found the link actually on how it works.
http://www.amazon.com/gp/feature.html/?ie=UTF8&camp=1789&creative=390957&docId=1001388121&linkCode=ur2&tag=greendcom-20
Pre-order an Xbox One and Trade In a Console, Get a $30 Bonus Toward an Xbox One Controller
For a limited time, while supplies last, trade in an Xbox 360, PS3, or Wii U console and get a $30 bonus credit toward an Xbox One controller.
How to redeem this offer:
1. Pre-order an Xbox One console.
2. Select a qualifying console from the list below and click the "Trade In" button.
3. Send in your item for processing following the instructions provided.
4. Wait for your item to be received and graded.
5. We will apply your Amazon.com Gift Card directly to your account in the amount of the trade-in value for your console.
6. We will send you a $30 bonus credit via email within three business days of acceptance of your trade-in.
7. Add a qualifying Xbox One controller from the list below to your shopping cart and proceed to checkout. On the final checkout page, enter your bonus credit in the "Gift Cards and Promotional Codes" box. Once you click Apply, the discount will appear in your Order Summary.
Not as good as the $30 off of a PS4 controller but still a pretty good deal for anyone that has an Xbox One on pre-order on Amazon.