I know the purpose of these questions. They're the most natural way for you (the player) to get backstory on the events surrounding you regarding the Mojave Wasteland and its setting
But judging from the timeline (see http://fallout.wikia.com/wiki/Timeline) of events in the Fallout universe we'll see that the first appearance of the NCR in the Mojave Wasteland is as early as 2270 with the NCR conflicting with the Legion as early as 2273. The events of New Vegas the game take place in late 2281 almost a decade later.
So, maybe from a roleplaying perspective if you roll (get it? dice?) that way, where has your Courier been for the past ten years that you don't know about either the Legion (so not out East) or the NCR (likewise out west) or atleast what is your rationale for the Courier not knowing this information above and beyond the plot-device of giving backstory to the player as mentioned above?
Or is it all just an acute case of bullet-induced amnesia?