They will have to buy the headset after the pre-order phase is done. Think of this like the Samsung S7 series, they have VR capable phones too, but they have to buy Gear VR when Sammy isn't throwing a freebie.
In a way, to justify the cost of the pixel, we can say we already paid for it. So we aren't getting it for free.This way, we don't feel bad for paying a premium price tag and we don't have to spend 150USD for a bluetooth headset on top of the already premium price tag.