This is actually standard practice among US wireless carriers. AT&T, T-Mobile and Sprint all require devices to be paid off before they can be unlocked; there's also an unlocking waiting period, ranging from 14 to 50 days after a customer makes a request. Verizon will not require customers to have paid off their phones to unlock them, and will continue to allow customers to use unlocked phones from other carriers.
Verizon claims that the focus of this policy change is scammers and thieves who target brand new phones. Unlocked phones are a valuable target because they can be quickly resold on the domestic or international market. The timeline of this change isn't clear, but Verizon said it will provide an update before the policy goes into place.
This change could really affect Verizon customers who travel internationally. Using an unlocked phone overseas is a standard practice for frequent travelers; it's simple to buy a SIM card and swap it out for your own. It's not clear whether Verizon will unlock the phone early at a purchaser's request once the policy is in place.