Attack on Austin IRS office - terrorism or criminal act?

When Timothy McVeigh blew up the Murrah Federal Building in Oklahoma City we agreed it was an act of domestic terrorism. When Umar Farouk Abdulmutallab, the underpants bomber, was caught trying to blow up Northwest Airlines Flight 253 there was no problem identifying it as attempted terrorism.

A few hours after the plane crashed into the Austin building housing IRS offices various officials met with the media to proclaim that the incident wasn’t terrorism but a criminal act. Some on the far right such as the anti-government "Patriot" movement, which includes thousands of tax protestors, elevated Stack to the status of hero and are only disappointed that the death toll was so low. Yet if Joe Stack had been of the Islamic faith there’s little doubt the incident would have been called a terrorist act even by them.

How is the motive for these acts really any different? How can mass murder of civilians be terrorism sometimes and a criminal act other times? How can that same act perpetrated by an American citizen against other American citizens be considered heroic to anyone in this country, are we that far into division? Doesn’t anyone recall Lincoln’s admonition that “A house divided against itself cannot stand”?