It is the GM that makes the story.
If the GM tells the players "You want to try to catch up to the horde of villains by traveling through the night without sleep? Ok, but you risk exhaustion." that is entirely up to the GM. It is his story.
What is he going to do if the players say "Nah, we sleep"? Let the story die?
All choices in any game are completely arbitrary. They are completely up to the GM. The GM is not forced to do put anything on his players that she doesn't want to happen.
I must say, if this is your view of the GM/player relationship, then you have played with some very horrid GMs.
@Gmazca is correct, the purpose of the game mechanics are to allow player choice. The GM referees those choices, and tells the story as the players play it, if the players choose to go after a horde of villains, that is their choice. If they choose to chase so long that they risk fatigue, that is their choice. The point of the rules is to explain the consequences of choice.
