There can be a separable sense of "should" that indicates rationality. Thus, "we should sign the treaty" can be an interesting truth for both parties when the "should" is that of rationality, and true for both parties but only interesting from the human side when the "should" is a moral should.

This commits one to what philosophers call moral externalism, namely, the view that what is morally required is not necessarily rationally required. Which is not a reason to reject the view, but I expect it will be criticized.