There is a difference between normative economics (which 
  the political leaders and Paul Krugman propound - ie the 
  "That's how it should be" class) and positive economics 

Well, sure, but there can be no obligation to do that which cannot be done. Let's say you believe that the Federal Reserve's QE and money printing activities are like monkeying around with the number of shares in the cap table, that this nothing to do with the process of wealth creation, and that they downplay the effects of ruinous inflation on economies (viz. Weimar 1924, Argentina 2001). Then the positive and normative are tightly linked. You should not do X (normative) because X will lead to disaster (positive).