If for some reason humans who liked to torture toddlers became very fit, future humans would evolve to possess values that resulted in many toddlers being tortured. I don't want that to happen, and am perfectly happy constraining future intelligences (even if they "evolve" from humans or even me) so they don't. And as always, if you think that you want the future to contain some value shifting, why don't you believe that an AI designed to fulfill the desires of humanity will cause/let that happen?