Why is a document describing the standards for social interaction that contributors pledge to live up to bad? Open source has been and continues to be rife with social interactions that are bad for individual contributors and for the project as a whole. CoCs strive to head that off and document a process for when people seem to violate their pledge.
> You don't need to tell people to be polite (and you shouldn't). If someone isn't everybody will notice it.
Unless it's done in private. If someone is a jerk to me in private (over email or Slack or whatever), what do I do?
If I'm contributing to a project without a CoC and someone is a jerk to me, I'm much more likely to just contribute to a different project, or start my own fork, or just stop contributing to open source entirely.
Projects with CoCs have a stated procedure in place for dealing with abusive contributors. That's what the whole thing is about. Of course you shouldn't have to tell people to be polite, but that's not how the real world works. There are jerks everywhere, and more often than not they're going to express that in private.