I'd like to think people who are online are reasonably able to read and think things through. This isn't always the case.
No, I'm not just commenting on Bush-supporters here.
I have a friend with a slightly unusual name. She got harassing messages in her LiveJournal because she shares the same name as a woman in another state who was involved with a murder.
So the lesson is that harassing people on the Internet is stupid. But it's beyond stupid to not realize that multiple people online could have the same name.