Freedom of expression is related to the advancement of populism because it allows populists to get their message out. You can see this happening in the last couple years with both the Tea Party and the Occupy Wall St people. These are both populist movements in a way and both have been able to advance (to the degree that they have) because of freedom of expression.
Populist movements are movements of the "little people." They depend on their right of freedom of expression to get their message out to others who might agree with them. Because the US guarantees freedom of expression, populists are able to advance their cause.