Yes, it's total private tyranny if the person who started the subreddit chooses to keep it that way. But if Reddit (the company) doesn't like the way you're running your sub, they can and will replace the people in charge.
That's kind of bizarre to me. They act like they're firing employees, and replacing someone for that role, when they're just random volunteers to a community set up by a person unaffiliated with Reddit.
The issue here is stuff like brigading and stuff against the terms of service like straigh out nazism etc. And even those subs stay long often.
Brigading is when members of sub coordinate and go to harass people on different sub or mass downvote their posts etc. The subs that have been closed or taken over were notorious for that.
