What these people don't understand is that doing this will not stop spam, it will merely make it legitimate for the large junk-mail houses to get into the spamming business. Worse yet, these people will undoubtedly get legislation passed to make it illegal to block spam site-wide. Take a hint from how difficult it is to block normal postal junk mail. The Post Office gets the majority of its revenue from junk mail. It certainly has no interest in stopping it.
Many companies get all their customers from paper junk mail.
They have no interest in stopping it. So, the politicians have no interest in stopping paper junk mail. Make no mistake about it, Internet spammers have exactly the same plans for Internet-borne junk mail. The saddest thing about spam, though, is that it would be preventable if it weren't for the naive or uncaring system admin. Of course, lost in the lobbying will be the fact that it'll be the end users who really end up paying for all this.