No, they do not all require explicit approval to keep out spam. In GNU Mailman, this is a config option: you can allow non-subscribers to post or not.
All of the mailing lists that I operate on my own mailing list server allow non-subscribers to post. Due to my anti-spam configuration, this isn't a problem.
Traditional mailing lists, before the rise of spam, were usually this way.
And anyway, this is a separate issue. A list which does not re-mail postings from non-subscribers can nevertheless not do Reply-to: munging. So once you are on the list and participate in discussions, you're still sending messages to the list, as well as directly to those in the discussion.
Earlier this year I was involved in a mailing list discussion in which one of the parties was actually (unbeknownst to me for a while) a "persona non grata": someone banned from posting to the mailing list. His postings were not being seen by the subscribers, but only those in the debate. This list does use Reply-to:; he just (trivially) circumvented it.
