Shifting topics back to the original post, I have little issue with alternate histories choosing an ending and finding choices that could have led to that result. There is no reason bias needs to pollute this scenario.

Once you know the conclusion you want, though, the human mind is very good at finding a way to make the details fit in between and make it feel like you're right. You're right that bias doesn't have to exist: there's nothing about having the conclusion first that inherently renders all hypothesized steps to reach it biased. After all, most sciences start out knowing the conclusion and have to work back to the source. But with alternate history rationalization easily sneaks in, so it's better to work the other direction so you can try to push the outcomes somewhere other than where they really ought to flow.