I pay for my mailbox space. Spammers are technically stealing from me by eating up my space without my permission. Maybe we could just charge them with unlawful taking?
The problem's not the content, it's the fact that we're PAYING for it, and that it's getting to the point where it's about to overwhelm the net into gridlock. A spam filter on your computer keeps you from seeing any of it, but it's still clogging up the lines to get to your computer, taking up space on your drive, possibly forcing you to go over quota on your ISP account and incur extra charges, etc. And when ISPs have to double their capacity just to handle all the incoming spam, those costs get passed on to the customers. It's basically the same reason Congress passed the junk fax law: It's your paper and ink they're using to spam you, and that's nothing more than simple theft.
But like I said above, none of this is going to matter when your mail stops going through at all. In the last year, I have seen a rapid and steady increase in the amount of time it takes for regular emails to reach their destinations. Email from friends that used to arrive with five seconds now take anywhere from five minutes to two hours. "Breaking News" email alerts from the various news channels sometimes arrive a DAY AND A HALF after they were sent, and almost never earlier than a half-hour after being sent (which hardly makes them "breaking" any more). This is almost entirely due to spam.
Some sort of legal and/or technical crackdown is going to be required around the end of 2003, because it truly is getting towards the tipping point.