Economist James D. Miller makes it on InsideHigherEd.com. 

A tax on elite colleges would reduce inequality. Students who attend top schools have vastly higher lifetime incomes than other Americans do. And even if the tax reduced financial aid and so increased student borrowing, it would still reduce inequality because those who graduate from elite schools with large debts are much better off financially than are their peers who do not attend college.

That strikes me as a wildly unconvincing reason to tax a non-profit. Then again, the question is really whether Harvard should still be considered a non-profit institution…..