To understand a bit of the outrage, look at the open source outrage over SCO claiming to have invented/owned Unix. We all know it's bogus, and as much as we might try to ignore them, they use the claim to try to stick their noses into our business.

Or, if you prefer, how about somebody who might start retroactively claiming that you or your ancestors were criminals and you're now living with ill-gotten gains. That kind of defamation can have legal consequences (and you have legal rights to sue for slander/libel, in response). While it doesn't directly injure me if you start saying that my mother was a hamster and my father smelt of elderberries, it's really something that I can legitimately take as an insult aimed at myself (i.e., it's a grown up version of school-yard taunting). While there may not be legal liability, it's still very poor behavior.