Philip Michaels:

Verizon wants more of its subscribers to opt for shared data plans which cover multiple devices like smartphones and tablets. To convince subscribers who’ve clung to their unlimited data plans to move to this new setup, Verizon is taking much of the choice out of the decision process: If you want to upgrade to a new 4G-compatible phone, you’re going to have to give up that unlimited data.

The carrot and stick.