i dunno much about all that, but there's still a difference
if someone signs me up to a spam list on one of my email accounts, i don't care that much
sure, it will be irritating, but i have all the emails i want to keep saved elsewhere and printed off
i have several email accounts
whereas sticking your IP address up for a load of people to see, and practically advertising that you're doing that (which is effectively what we're doing when we sign up, as everyone knows about the admin stuff) can cause problems less easy to deal with