Without starting a huge debate over corporate greed, an open phone enables those who root to circumvent VZW controls for things they hope to monetize (like tethering).
A better example might be that, while VZW gives you unlimited data, they don't really intend for you to suck up 10 or 100 gigs a month. They don't intend for streaming hi-def video or torrents to be part of that without you paying additional. So it's a VZW service you'd have to pay for, except again people who root might be able to circumvent that.
Now, whether that 1-2% of people who root is worth fretting over, well that's up to VZW. The goal is most likely for it to be sufficiently hard to root that they don't have to worry about that 1-2% becoming 15-20% where it does start to significantly impact the bottom line.
