It's not just the subculture. It's like any trust game. You violate trust, you won't get trusted in the future. There's long-term reputation. And that's just so

It's not just the subculture. It's like any trust game. You violate trust, you won't get trusted in the future. There's long-term reputation. And that's just some of the factors behind how subcultures come to basically honor agreements like this. The concept of such agreements would be discarded and laughed at in a context where they were rarely kept. Just knowing that the concept exists is enough to know that it must have gained trust and deference in the subculture. Listen to the This American Life episode linked elsewhere in the comments to get the context. The guy who broke his promise actually meant the promise when he said it and then felt privately ashamed of breaking it (and breaking the promise was itself a spur-of-the-moment, wasn't-thinking decision). That's not what happens if it's just mind games.