Glen's characterization of the economic way of thinking as Goals filtered through Constraints leading to Choices makes for a nice set-up for extending economic analysis to "non-economic" choices. Right now, after walking them through how that framework applies to consumers and producers, he's having them think through how you can understand political action using it as well. His bottom line, as it is with economic choices, is that we cannot make the world submit to our wishes. We always face constraints and we always must make choices, and those choices have costs.
Making these sorts of public choice arguments to libertarian students is like shooting fish in a barrel, at least in the sense that they are strongly predisposed to being cynical about political actors. What public choice can do is to give them a theoretical framework for understanding and explaining the perverse outcomes of the political process without assuming that politicians are irredeemably evil.