I wouldn't say she slips in not saving them - I never got the feeling that Buffy is supposed to save everyone - she saves the world, and the people in it, but she is quite happy to lower the death rate in Sunnydale High - and doesn't cry that she didn't save everybody in it. She is just one dirl in the world, after all - Faith nonwithstanding, so she cannot be in every place - So, when she is confronted with "You didn't save us!", her answer is "tough luck, get over it", not "my bad, woe me". With Cassie it is different, of course, because Buffy here did everything to save her, and it still wasn't enough.