>>13828452Because +1 keeps the sign in multiplication and -1 doesn't.
If I gave you 2 apples for every apple you have, and you have 2 apples, I give you 4 apples.
If you owe me 2 apples for every 2 apples you have. And you have 2 apples, then you owe me 4 apples.
If you owe me 2 apples for every 2 apples you have. And you owe me two apples, then I give you four apples, because you don't own any apples instead you "owe" me two which is the opposite, so I have to give you two instead.
