In an open letter to their professor, Greg Mankiw, the students explained that their actions were driven in part by a desire to show solidarity with the Occupy Wall Street movement.
A legitimate academic study of economics must include a critical discussion of both the benefits and flaws of different economic simplifying models. As your class does not include primary sources and rarely features articles from academic journals, we have very little access to alternative approaches to economics.
Harvard graduates play major roles in the financial institutions and in shaping public policy around the world. If Harvard fails to equip its students with a broad and critical understanding of economics, their actions are likely to harm the global financial system. The last five years of economic turmoil have been proof enough of this.
On his blog, Mankiw notes that the students' actions are ironic because the class topic for the day was income distribution and the growing gap between the top one percent and the bottom 99 percent.
Mankiw was the chairman of the Council of Economic Advisers under President George W. Bush. He is now an advisor on the Mitt Romney campaign.