Supply and Demand. The number of consumers using smart phones is increasing by the day. This means 2 things.
1. Increased usage. If you give everyone unlimited data, you need to worry about what your servers can handle. By limiting data, you discourage overuse.
2. Usability. Nobody wants to spend hundreds of dollars on a new smart phone and not be able to use all of it's features. Cell providers know this, and know that people will spend the extra money to get the most out of their phones.
It sucks, and will surely outrage lots of people, but Verizon probably feels that more people will just accept it rather than going to another company.