Most people accused of sex crimes (or any other crimes) don't have access to "the best defense they can get" - what they're given is some overworked mediocre hack that they can afford or that the court appoints, while people like Epstein use their political connections and vast wealth to dodge evade the law for years. That's Dershowitz's notion of "justice."
Presumption of innocence is a keystone of our judicial system. Dershowitz is just being consistent.
It's pretty easy to adopt the "principled and consistent" position when it benefits you, your friends, and associates.