I understand it. (I don’t agree with this point of view, but I understand it.) It’s not so much that the people who aren’t convinced don’t think that Bush said things that weren’t true, but that they’re not sure that Bush did it intentionally with malice aforethought and that the end result was a bad thing.
There are two aspects to the statement, “Bush & company lied to us, and got us into a war we shouldn’t be involved in.” (You didn’t make the second part explicitly, but if you thought it was a just and justified war, you most likely would not be as upset about it.)
The first is that they lied—which is a very loaded word. To lie, you have to know the truth and tell a falsehood with malicious intent. If you say what you believe to be true, even if it’s false, it’s not a lie; if you say something you know to be untrue, but there’s no malice in it, it’s not always considered a lie. So to demonstrate that Bush & company lied about things, you have to show that they knew that what they were saying was false and that they said it with malicious intent. That’s a much higher burden of proof than just showing that they were wrong. Even in retrospect, when we know that Saddam Hussein did not have great military resources or weapons of mass destruction and that Iraq and al-Qaeda had no significant ties, if you want to convince someone that Bush & company lied, you have to show that they knew that at the time—not merely that someone in the State Department knew it, or that there was “no convincing evidence,” but that Bush & company knew for a fact that what they were saying was untrue. This is difficult to demonstrate for those of us who don’t like Bush and his cronies; if you are inclined to give him the benefit of the doubt, it becomes impossible.
A lie also requires malicious intent, and that ties into the second part of the statement. From your point of view (and mine), this is a war we should not be involved in. There are people out there, foreign policy “hawks,” who think that the best defense is a good offense, and Islamic totalitarianism, whether fundamentalist or secular, needs to be rooted out at every opportunity, either as a preemptive strike because they’re planning something or as a retaliatory strike because of 9/11. (The lack of connection between Iraq and al-Qaeda is irrelevant; in this mindset, they’re all Islamic, they’re all terrorists, they all hate us, and they’re going to attack us at the earliest opportunity.) These people, in addition to giving Bush the benefit of the doubt because they support his actions, are likely to see the “lies” in a positive light: not intentional falsehoods with malice aforethought, but necessary fictions that had to be told to get everyone to support what had to be done.
There are two schools of thought in foreign policy: one is the idealist camp, which holds that the best route is to invest heavily in cooperating and working together to avoid conflict in the first place, and the other is the pragmatist camp, which holds that the best route is to ensure that when conflict inevitably occurs, we are on the winning side. The “Bush lied us into war” viewpoint grows out of the former approach, while the “it’s a necessary war, and the lies are irrelevant because we did the right thing in the end” viewpoint grows out of the latter.
As a thought experiment, if you start with “We’re not going to be able to avoid conflict every time, so we need to make sure that when it happens, we’re on the winning side” as an axiom, you should be able to reason yourself to the point where you don’t think it’s important that Bush and company lied.
(I also think this is a useful technique for arguing: if you can only conclude that your interlocutor is crazy or stupid or demented, you can’t figure out how to change his mind. If you can, on the other hand, understand why he comes to the conclusions he did, then you can figure out how to bring him around to your point of view.)