When someone joins a context you are operating in, whether it’s a community, a workplace, a team or some other social situation, they will have to find out how to navigate that context.
Commonly, there will be no boundaries or expectations set, and the new person will have to find out what is successful or not in the context by themselves.
Providing clear boundaries, ie. We don’t do that here, becomes an act of kindness, as it removes some of the burden of trying to understand the rules of engagement from the new individual. It is in many ways, a method to Set someone up for success.