When using the scenario cards (which I highly recommend) you may find yourself without enough politics cards to go around. I think this is possible even at a lower player count than 5.
But, if I just reach over and ring the bell of game developer summoning, I suspect Seth will be along shortly with a more authoritative answer.
That's correct, the extra Politics cards are for the scenarios, and the 5th player of course.
In the new expansion (coming out any minute now), there are a bunch of Politics cards, because of the new Politics role