I have mixed feelings on this. A series is more fair, but the excitement from a must win game is fun. At the very least I think they should be honest and not call the WC teams playoff teams. If KC had lost against the Athletics I wouldn't have thought of them as making the playoffs. It's a play-in game, nothing more. Just like the NCAA doesn't really start with the Tues and Wed play-in games.
The eight teams are split into two, four-team, double-elimination brackets, with the winners of each bracket playing in the best-of-three championship series. (source: Wikipedia: College World Series)