Castro didn't realize warm up pitches were done, and he missed an actual game pitch. That is obviously wrong, but even if it happens once every year it isn't going to matter.
So, as if we were still in high school, Quade benched Castro for an entire game as punishment and lets a lesser player play. So we are missing an entire game of Castro now. Which is worse?
Some things that are worse than not paying attention to one pitch:
Making an error
Hitting into a double play
None of those things would require punishment. The difference is that these are all a lack of skill while Castro had a lack of concentration. All are flaws, and of these Castros was the least harmful.
Quade didn't punish Castro, he punished the team and the fans. What if the ticket you bought was for the game Castro sat? Then the Cubs go onto lose a game where they stranded runners almost every inning. 1 or 2 hits could have been the difference.