Reddit is just a platform. If mods of a subreddit are seen as being heavy-handed in their moderation then people a free to create a competing subreddit.
For one, the worst behaved subreddits moderation wise are often the "default" ones, meaning the ones that everyone sees when logged out. They have a favored position on the site, in other words. Every user is subscribed to them by default, which also means their content is going to wind up on /r/all more often.
Also, once a subreddit gets that size, crap mods can get away with a lot more because the volume of submissions make it hard to tell that something untoward is going on. And the people who pay attention (perhaps a few thousand on a good day) will easily be drowned out by those who do not.
Community forks happen, but never at that level. The deck is hilariously stacked towards the entrenched communities.
I don't think the parent was saying that you shouldn't take over a subreddit in response to moderator behavior. The thread has been saying two seperate things: that you shouldn't try to take over a subreddit for how its users behave, and that inappropriate moderator behavior is a problem on reddit. If you can convince a subreddit's subscribers that the moderators are shit and you'd do a better job, then you should certainly take control of the subreddit.
Usually it's easier to just make a new subreddit and convince people that yours is better.
