Why would Sprint text me a link for a free iPhone 5s that is not entirely free?

1. I received a text message from Sprint stating that I qualify for a free iPhone 5s.

2. I called Sprint's 1-800 number and confirmed with them on three separate occasions the phone was free and there would be no change to my bill.

3. They sent me the phone and upon activation tell me that my bill will go up $15 plus tax.

Obviously, I returned the phone.

Added (1). The charge would make my bill go up $15 per month plus tax.

Sales gimmick. Plain and simple. They want your business.