Data is not just data. People who currently use 100MB a month in data checking email and browsing the web could suddenly increase that exponentially by making video calls. It isn't a matter of the customers being able to pay for it, it's a matter of the network being able to handle it.
Combine this with the loss of revenue as people use less voice minutes, downgrading or completely eliminating their voice plan (it's possible here in Canada at least), it could severely hurt the company financially and operationally.
As for being able to currently use VOIP, there is a huge difference between a third-party app such as Skype and a native app pre-installed on every phone, and "Facetime with
" throughout the OS (in the messages app, in contacts, etc). People who would never think to use a third-party app quickly and easily become familiar with Facetime.
They need something to actively discourage people from using Facetime.