In a way, that's a big part of what I see as their largest problem - they are relying on the zeal of the subreddit moderators to work for free. (They should have a model which allows moderators for large subreddits to get paid. And for the subscribers to vote for moderators.)
This is why the large subreddits are so incredibly polarized in either left or right directions - the moderators are not paid to deal with things fairly, they are there spending their time working free - the only benefit they get is that they get to enforce their particular political choice onto others in their subreddit.
Look at e.g. r/politics and r/the_donald as extreme examples of what I'm talking about.