This perfectly ordinary signature makes me liable for any message I send to someone I don't already know. A friend of mine tells me Joe Blow at Acme Computers might need a technical writer, and I need to pay the rent. I do technical writing at reasonable rates. The Smith bill makes this email illegal. Someone using a forged From address emails fifty million copies of an announcement that the Second Coming is nigh and everyone must accept Jesus as their personal savior.
Mail servers crash under the load.
A political campaign mails a hundred thousand solicitations for donations. Even though it involves money, this spam is not a commercial advertisement by the Smith bill's definition, so the law can't touch it. But since the spam itself does not contain an advertisement, it probably isn't illegal under the Smith bill. Something is wrong with this picture. Instead, it takes the politically easy way out by banning unsolicited commercial email, whether that email is spammed or not.