In answer to your thoughtful question, it might be because, at this point, the Oregon Ranchers have said they want to resolve the issue peacefully, they haven't killed anyone, cops or innocents or otherwise, and they haven't destroyed property, burned buildings and cars, assaulted innocent bystanders, or killed other people because their lives don't matter. That's why.
If they start doing these things, we can revisit the terminology and be prepared to up the stakes by labeling them thugs, at least if they begin destroying things or start saying they want the Feds dead. And if they begin randomly killing innocent people in the name of their cause, why not? Let's do what we never do with that religion we're not allowed to criticize and actually call them terrorists.