Ebo, in the past there was an idea not to sign points. Wether they are negative or positive. I don't know if many people follow this direction. It certaintly has some advantages for the overall balance of the aps system.

You could also wonder why people do this? Personal vendetta's like Allenb said?
And yes this abuse happened before. I think one of the first big cases was when someone signed with Ennis name.