That only works for the top few dozen subreddits, the ones that were once 'default' subreddits, they can't afford to pay people to sit and moderate the other hundred thousand subreddits all day long.
They probably don't care about that long tail. While valuable to individual users, I'm sure their business is an 80-20 like any other. For monetization, they just need to make sure the default subreddits stay moderated. Which they can with hired guns if required. This is how every other social network works after all.