Spamhaus is the most rabid group of extreme anti-spammer teenagers out there (or at least they act like teenagers). I've had various SMTP servers (for various companies) blocked by them, usually for very questionable reasons. You used to have to argue with them on their forum (and take a beating from all of the kids on that site) before they'd remove you. The result is that SMTP admins get it from both sides: Spammer…
I see you've never met SORBS. Or even NANAE (news.admin.net-abuse.email).
Blacklists are a pretty powerful tool to get ISPs and the like to do something about their spam. And Spamhaus is usually right. And yes, I've been on SORBS (due to some other account in the same /24 spamming at some point in the past...)