Here's a good reason why forging e-mail headers should be considered identity theft. It can ruin someone's reputation, career, or sleep.
Personally, I don't care if it's made up addresses that most spammers use. The idea is that they're pretending to be someone else when they send those e-mails. (Ignoring the fact that, occasionally, they'll accidentally use the address of a real person.) (Okay, including the fact...)