Comment 2 for bug 266269

Because they're not signatures they are included by MUAs in every reply. That reply typically goes back to the list and another list footer is added, that also then gets quoted. See http://lists.edlug.org.uk/pipermail/edlug/2017-February/000431.html

The list footer should be intended to be a signature. If the user has a signature then that will precede it and just increase the one signature's size and it will all be stripped in the reply, which is desirable. Many newcomers to mailing lists aren't aware of the signature convention so Mailman should default to using one and explain what it is and why.