Of course you are right Buffy was so in love with Spike she spent a previous evening grasping a cross and her bed surrounded by garlic and yes telling someone no and trying to stop that someone sexaully assaulting you... is of course a form of self abuse , just like cutting your hair hoping your Stalker would find you less attractiveCutting your hair isn't an assault metaphor or not just. People who slept with the enemy in wartime were often forced to cut their hair and in real life a lot of women cut their hair after the end a realationship it's a statement of making a new start. In Buffy's case it can function as both a shame and a severence with a past that makes her feel ashamed. I know your reading is abuse but to me it's more a drug or especially accurate a self harm metaphor. Buffy uses Spike (and although he's certainly not blameless she does despite knowing how he feels) for pretty much the same reasons people suffering from depression (like she is) self harm just so she can feel something. We all read things differently but yours isn't other people's or the author's intent no matter what you think and feel and how obvious it is to you.
Im sticking with my first assumption , crap writing, just like the demon eggs were used to break up Buffy and Spike the lighter was used to as an excuse to show how easily Spike could manipulate a mentally ill Buffy which carried on throughout S6 until the attempt rape , when Buffy was mentally strong enough to stop him