That's a good question. Part of the answer is that a properly run mailing list doesn't require subscription in order for you to post a question or a patch, and also keeps you in the loop (i.e. lets you receive the replies to your posting) by avoiding "reply-to header munging". The barrier of having to subscribe to lists is just a quick and lazy anti-spam measure that decreases usability for everyone.
So that is to say, mailing lists in their original inception didn't require "tracking" as a prerequisite for engagement. They were really just re-mailing robots: you write a message to a robot, and it sends it to others. Those others do "reply all", so that you receive the reply even though the robot doesn't have you in the list. The robot stays in the loop because it is CC'd, and so the list subscribers can track the discussion.
When I have some question, or want to report a bug, I don't actually want to track all of the activities in that project's mailing list. It is rude to expect that of me. And anyway, there are web archives of mailing lists!
All the mailing lists that I operate are in this classic open manner.