The FBI says:
"Domestic terrorism" means activities with the following three characteristics:
1) Involve acts dangerous to human life that violate federal or state law;
2) Appear intended (i) to intimidate or coerce a civilian population; (ii) to influence the policy of a government by intimidation or coercion; or (iii) to affect the conduct of a government by mass destruction, assassination. or kidnapping; and
3) Occur primarily within the territorial jurisdiction of the U.S.
#3 we have.
#1 - I assume they're violating some laws, yes. But have as yet committed not acts that were in and of themselves dangerous to human life, have they?
They certainly hinted at violence. Must we wait that they do? The weapons they have aren't there for them to collect and trade.
#2 - The folks in Oregon are not trying to intimidate civilians, as there are none present. There's been no mass destruction, assassination, or kidnapping. So, the only possibility is that it falls under ii - influence the policy of a government by intimidation or coercion.
A bunch of folks holed up in a building are not terribly intimidating to the government.
Success or efficiency is irrelevant. It is clearly their intent to intimidate and impact policy.
So, that leaves us with coercion. That is the practice of persuading someone to do something by using force or threats. The only force or threat as yet applied has been of the form, "if force is used on us, we'll use force on them". That doesn't seem a solid claim of coercion against the government on the land policy they say they want to change.
If they are so peaceful and not dangerous, not ready to die and kill for their cause, why not go and arrest them for their illegal activity? The FBI could go inside unarmed and just handcuff them. Right?
Simply put, nobody's really scared or intimidated here.
Intent rather than success is important. An incompetent terrorist is still a terrorist. Seeing as no federal agents has tried to arrest them yet, I think they managed some intimidation.