For one thing, the grad student's piece reflects a fundamental disdain for the sovereignty of the USA. Buchanan's piece does not. For another, the grad student says the USA is quite terrorist itself. Buchanan's piece does not.
I admire Buchanan for his courageous contribution to the public discourse. He's not "blaming" the US for the particular events of Sept 11; nor, by the way, was Jerry Falwell "blaming" God or the US for the murderous choices made by the terrorists. To me, Buchanan's main point is that we have gone out of our way to make enemies abroad. In his talking head appearance opposite Newt Gingrich last night, he elaborated on this, using the recent Yugoslavia intervention as an example. Falwell's main point was that it is God's hand that restrains evil in the world, and we should strive to be as Godly a nation as we can.