The unwritten rules get written
…when someone decides to selfishly push.
There’s an assumption of civility and fairness in all of our interactions. When a harsh competitor unilaterally breaks unwritten rules (because it’s “not technically against the rules”) the community then writes down a new rule.
The best way for a market to be a free market is for the participants to exercise self-restraint.
The second best way is for clear and useful rules to be stated and enforced.
What doesn’t work are unwritten rules that are often broken by selfish bullies.