From personal experience, I can tell you that kissing someone like that, knowing that he has feelings for you, and you have no feelings for him, isn't gratitude, it's cruelty. Buffy knew that Spike had feelings for her, the bot was evidence of that, and she didn't return those feelings. Spike might have been a vampire with a century if sexual experience, but he was also a man who had most likely died a virgin, who had almost certainly never had any sort of real romantic experience. There are better ways that Buffy could have expressed her gratitude, like, maybe, taking him back home with her, taking care of his wounds, and bringing him fresh blood. She should have shown him that she was grateful, not given him hopes for a more intimate relationship.