Aside from a small number of general site-wide rules, reddit is based around the idea of "your subreddit (meaning, you're a mod of it), your rules". And nothing about it requires that every single rule be exhaustively defined up-front; moderator discretion is ultimately the sole definition of a subreddit's rules.

If you don't like a subreddit's rules, you're free to create a competing subreddit on the same topic but with different rules, and try to attract people to it.

I moderate a medium-sized (200k-ish subscribers) subreddit, though, so I know how unpopular this idea is with some people.