Signed binaries don't stop malicious binaries. It can help prevent the spread of malicious binaries once they've been identified as malicious, but it does nothing to stop the initial mayhem.
Since you have to sign with a key that you have payed apple to authorize. Apple has the payment details that connect you and the signed binary together. In addition the membership has licence conditions that in turn allow Apple to sue you.
The intent is a deterrent effect.
Though this morning I realized I misunderstood the point of the grandparent here. He was saying you can have a signed binary that is still malicious. I thought they were talking about what apple does in the case of unsigned.