Because Robin thought it was a ‘checkmate’ case that blackmail was good and should be legal.
How exactly Robin comes up with his weird ideas is a mystery that scholars have written many dozens of books theorizing about, but we have learned to rule him in as a thinker who comes up with excellent ideas, and engage with them substantively even if they seem obviously wrong (as was true for Zvi in this instance).
Because Robin thought it was a ‘checkmate’ case that blackmail was good and should be legal.
How exactly Robin comes up with his weird ideas is a mystery that scholars have written many dozens of books theorizing about, but we have learned to rule him in as a thinker who comes up with excellent ideas, and engage with them substantively even if they seem obviously wrong (as was true for Zvi in this instance).