The conference makes a lot of money on that game, which in turn goes to the teams in the conference. It is to their advantage to play those games. Also, Clemson got in because of the CCG. No game and the ACC is a one-bid league. If SMU was bounced because of it, Clemson gets in because of it and the ACC is no worse off for playing the game.
I get that it's a 13th game, but there's not a huge difference between 12 and 13 games and the committee should take the entire body of work into account. SMU lost to BYU at home. If they had played in Provo I don't think that game would have even been close.