I'm not sure why but usually it's academics who send these. Maybe people sign them up for mailing lists b/c they are jerks or something. They all seem to be sure that they've never ever signed up for a mailing list.
ISP sends me email about the compliance issue, I put the email address into a form that blocks it from ever being associated with any of the lists on that server, and it spits back an email detailing when they signed up, confirmed, etc, that they've been blacklisted, and then attaches a jpeg image of the headers / footers with unsub links circled in red so the ISP can close the case.