No you don't have to buy it from the.
It is tied to the 3 months free, $10 LTE plan and waived activation that Apple announced. So in the case of Verizon it is a $60 deal ( 3x10 free months, $30 Activation waived ). All 4 carriers in the USA are offering and equal deal with minor adjustments. For example AT&T charges $25 for Activation so their offer is worth only $55 dollars but is equivalent to Verizon's.
Dave