The trade gives you $100 credit toward the purchase of your new phone which is why it cuts the price of those three models in half. AT&T is telling those who are trading in their current phone to wipe it of all personal and sensitive data and restore it to the original factory settings.
If you've been thinking about trading in your current phone for one of the three participating handsets, AT&T is not only subsidizing the price of the phone, but is also giving you $100 credit toward the new model. Sure, you might be able to get more for your old phone by putting it up on eBay, but that is a crap shoot. The deal from AT&T makes it all easier.