Public opinion is presumptively an input to policy formation in a democracy because politicians respond to it or at least are believed to respond [to it].
The case for trade is not just monetary, but moral. Economic freedom creates habits of liberty. And habits of liberty create expectations of democracy.
Democracy is the form of government in which the free are rulers, and oligarchy in which the rich; it is only an accident that the free are the many and the rich are the few.