I agree with you that it's terrorism of a sort. I think the reason the Obama administration said it wasn't terrorism (besides not wanting to use the word itself) is because this guy Stack wasn't looking to instill 'terror' within the populace, rather he was looking for revenge on just one group of many he felt had wronged him. He wanted to go out with a bang (unlike McVeigh who wanted to remain alive to continue his rampage of terror) so Stack committed suicide in the process.
He picked the IRS, but he also hated organized religion and could have chosen a church; he hated capitalism and could have chosen a bank or other large business. But once he offed himself, there's no reason to fear it will happen again -- unlike with the jihadists who are often quick to claim credit and threaten that more is coming (thereby terrorizing the populace).
When Hasan killed at Ft. Hood, I thought that was a form of terrorism also, but he's not been labeled a terrorist (he killed for political/religious reasons), and I think at least one group has claimed him as their own.