The first obvious fact: Faith and Spike are both murderers, who have done a lot of evil to people within Buffy's social circle, but even more evil to people outside of Buffy and her social circle. Faith didn’t kill anyone we, the viewers, actually know. Neither did Spike. That's fairly typical for BtVS. Anya didn't kill anyone we know, either. Andrew is an exception, since Jonathon was a sympathetic character whom the viewers now, although he was Andrew's own friend and not a member of the Scoobies. Willow kills someone we know, but Warren is total sleaze who killed her girlfriend. Angel killed someone we know, Jenny Calendar, but the narrative encourages us to see souled and soulless Angel as two different people. I think the show makes more sense if you view Angel and Angelus as the same person (both more sense in terms of Angel's psychology and more sense in terms of the mythology of the show), but both BtVS and AtS are both inconsistent in that regard, and the narrative encourages a certain amount of distance between the two that doesn't exist with any of the other murderers on the show, including other vampires like Spike or Darla. By AtS, Angel even starts using different names depending on whether he has his soul or not.
And just in case I haven't disclaimed enough (and unending fannish feuds seem to make such disclaimers necessary), I want to emphasize that I am not saying that I think, on a Watsonian level, that Spike is better at taking responsibility as a souled being for his actions while soulless--that seems to depend largely on what storyline you are looking at, and sometimes Angel is better at it than Spike--but that there is a narrative distance between Angel and Angelus that does not exist between Spike and, uh, Spike.
Or between Darla and Darla. I don't think it's a coincidence that in Angel S3, when there’s a storyline about Angel and Darla murdering an innocent women and her children, and we see, in flashbacks, the full of horror of it, Darla does not survive the storyline. Darla, who does not have the same divide between her souled and soulless self that Angel does, commits suicide after expressing remorse for what she did to that family. Darla dies, and for once, stays dead.
Faith and Spike do not kill characters we know. They do not kill characters that are beloved by the Scoobies or Angel’s gang (even Andrew didn’t kill anyone beloved by the Scoobies). It would be harder for the audience to sympathize with them, and harder for the Scoobies and Angel’s gang to forgive them, if they did. But when it comes time for these characters be the center of a redemption story, a human face is needed to represent the victims. One way of dealing with is problem is to have a character like Holtz or Wood introduced who lost loved ones to them. That’s one of handling it. Another way of handling it is to could use major characters who have been hurt by the characters in need of redemption in non-fatal way to represent the victims. You know, like Buffy Summers.
In “Sanctuary”, and in early S7 (from “Beneath You” through “Never Leave Me”), Buffy (along with Wesley, for Faith) represents the victims of Faith and Spike respectively. In the church scene in “Beneath You”, Spike conflates earning forgiveness from Buffy with earning forgiveness from all the other people he wronged (“And she shall look on him with forgiveness and everybody will forgive and love… and he will be loved.”). This sets the tone for the next several episodes. In “Help”, “Seeing Red” is treated as soulless Spike's worst moment (Spike even says, "I hurt the girl", as if Buffy were the only girl he hurt). In “Never Leave Me”, Spike's other victims are discussed and he explicitly says the Buffy got off easy compared to his other victims, but she is still used to represent all his victims.
But the things Faith and Spike did to Buffy are not as bad as the things they did to people outside Buffy's show. The things they did to Buffy, are horrid, but when the show treats them as the worst things they ever did, or as representative of all their crimes, it makes them both look much better than they actually are.
On the other hand, Buffy has to represent the innocent victim. Which means that she is portrayed as forgiving and taking back Spike and Faith, while her own crimes towards them are downplayed, just as their crimes towards people outside her circle (or, in some cases, inside her circle--i.e. Faith's assault of Xander which more or less got dropped by the narrative) are downplayed.
So that's my interpretation. I think the narrative choice of sometimes using Buffy to representing all the people wronged by Faith and Spike makes Buffy look better than she actually is--and makes Faith and Spike look better than they actually are.