Because even if it isn't under contract if you buy a new phone outright you can 99% of the time put it under your existing account.
I even asked two Verizon people at their 800 number line, they said that you could buy one outright and put it under your 30/unlimited account.
But why would you want to do that? I understand the need for unlimited data, but when buying the phone at full retail value, will you ever recover the $$ you spent on the phone via saved $$ on data?