Let me say up front that I share your philosophy in regards to the members of my clan being available for challenges. If they want to be available, great. If not, that's okay, too.
However, I do see the value in such a statement. I think it's important that newer members of the site understand that they can be booted from a clan by the leader without recourse to appeal. Perhaps the wording of the statement could be tweaked. Or, better yet, perhaps the message could be altered by the clan leader so that it becomes more clan-specific.