Some rules mattered, sure, but working a memory spell, that didn't break any rules because there were no rules about spells. Anyway, it was of the good because reducing conflict, that meant less stress, and everyone knew that cortisol and epinephrine were bad for your health. So, really, she was helping.
And sneaking the Lethe's Bramble into her pocket, that wasn't really stealing. It wasn't her fault that Anya was nosy and loud and couldn't keep a secret even if you paid her. Well, maybe if you paid her. But this was private, Willow's alone. Nobody else had to know.