This is tricky. Generally the spammers are doing it because they're paid to do it by someone. Penalizing is the right thing to do. But you raise an interesting point - perhaps an unscrupulous firm would unleash a spambot pointing to their competition, to damage them. I'm not sure of a good way for Google to differentiate, other than your suggestion of ignoring.

Thank you for allowing me to talk myself into a circle. :-)