This is just something that occurred to me, not an argument pro or con either way, but, of all the people in her life that Buffy has cared for and lost, Spike is the only one for whom she made any special effort, much less "kicking down doors," to assure herself that, yes, under Joss Whedonesque law, death is simply a temporary thing, and Spike was, indeed, back, healthy, and more or less happy, with his own mission.
And maybe, just maybe, when she found out that that mission involved making (un)life a living hell for Angel, she decided to just stand back and let the boys have it out, once and for all, whether or not any kind of oil was involved.