I wish TGGP was still around to expound on his Ass-Kicking Theory of Morality. But to sum it up (although I know I do it no justice) - animals have no moral weight because they have no ability to hurt us.*

Isn't this the same reason we're so worried about creating a FAI? Because we know that we won't have ANY ass-kicking-ability vs a GAI and therefor our moral weight to it will be nil. We have to impose our values onto the GAI before it is beyond our ability to hurt.

(*For simplicity's sake this ignores the moral weight that humans can give to certain animals (pets, livestock, etc) by threatening to use their own ass-kicking-ability to support the chosen beneficiaries)