Consider this. They are playing a match which will decide whether or not they make it past the Group stage and have a chance at winning Worlds.
If they did not ban Aurelion Sol, and lost, it would be devastating. No team wants to risk losing so much for so little.
If you watched the first game between CLG and ROX, you would know that they already tried letting Huhi play Aurelion, with disasterous results which led to them playing this final match to get out of groups. There is no way that they would let that happen again.
Being the best at League isn't just about being good on one champion, or even about one player being the best. The whole team has to be the best, both in terms of strategy and mechanics. And yes, sometimes being the best means being humble enough to admit that you are unable to beat a particular composition from the enemy team, and banning it out, just as they would yours.