These people have invited the email, and the associated expense, by placing a public email-to: button on their public www page.
Most people put up an "email to:" button on a public page for communications related to what's on the page. It's quite a stretch to assume that this invites any and all email from anyone who cares to send whatever they want. I can't say I've ever seen one that said "Please send SPAM(tm) to:" ... However, since others may think like you, I guess I'll have to add a line above my link stating that email not related to the purpose of my site will be happily proof-read at the rate of $200 per hour, 1 hour per 60 lines, minimum. Perhaps I'll get lucky. If I get a big enough chunk of SPAM(tm), it might be worth the costs of breaking down the anonymity. Until know, I'd never really figured out what causes lawyers. :)
A more acurate analogy, and to the point, would be if a business sends you a postage pre-paid business reply card, that is blank, and invites your comments on the card. They can hardly complain of the expense when people actually send it in, even if they don't like the comments.
No, this b.s. is more like having someone put a dead skunk in my mailbox, with no return address, trying to prevent me from sending them 100 dead skunks as a return favor. And about as welcome. - r.w.