I don't know. I think in most cases it has more to do with fan preferences than anything else - witness Angel fans who lambaste Spike for the AR but forgive everything Angelus did, or Spike fans who forgive Spike for the AR but won't excuse Xander in "The Pack."

I guess the closest comparison to Willow would be Angel's deal with W&H, and I'd say the breakdown of acceptance vs. condemnation in fandom is about the same for both characters. (Although it might skew opinions a bit that "trying to save your son" is somewhat of a better reason than "trying to avoid an argument.")