It probably isn't, but frankly if it's only during a LAN game where everybody is in the same building I see no moral dilemma with it. As long as you're not playing on someone else's account while that person is not present it should be allowed.
Besides this way the 5th player gets to experience CiV in full and if he likes it he might be pursuaded to purchase it after all. The alternative is that either player 5 is left out of the LAN from now on, or they ditch Civ for a game that they all have, neither option benefits Firaxis.