you don't have too assume that we have no choice in the future. you are going there to see what happened, what choices were made and why they were made (enter matrix music). i think that if they were gonna attempt time travel they wouldn't try to do it without making sure that the method of time travel was self contained. you wouldn't want to try to go to another time and hope that bill isn't on his lunch break when you are set to arrive.

on another note you could still travel to the future even if it was set in stone, and we had no choices. you would simply need to understand that you were supposed to travel to the future and that by doing so you are following the "plan" so to speak.