Excellent, something I have wondered about a lot, and you really gave such wonderful reasons for the different attitudes. It really makes a lot of sense.

Really in season seven, Spike does get away with more from the scoobies, maybe because of Buffy. But when under the Firsts control, he is taken back to Buffy's home, and Buffy rescues him from The First, but in Anya's case, Buffy is set to kill her for what she did in "Selfless". As Anya says Spike has a "Get out of jail free card", which seems to be Buffy's attachment to him.