You can fanwank it and say that the reason he was helping Dawn resurrect Joyce was because he didn't want to see BUFFY so upset, and losing her mother, that's the lowest place she's ever been and he wanted to get her out of it. She had no one to turn to, she was all alone, and Spike wanted to give her her mother back.
When Buffy died, Dawn had Willow, Tara, Xander, Anya, Spike and Giles to turn to, and while it must have been a terrible time for her, it must have helped that they were there. As far as the Scoobies knew, Buffy kept everything bottled up and kept it to herself. They didn't know that Angel was there to help her through it, Spike might have just assumed that she didn't feel like she had anyone left, so he thought the best way was to give her back her mother.
I also think that another part of it was that, no matter how Joyce came back, she was Buffy's mother, and he didn't think she would actually want to kill her, so that part of it never entered his head. With the Scoobs, Spike was aware of the risks of Buffy coming back wrong, and he knew that the Scoobs are much more black and white about something like that. If Buffy came back wrong, they would do whatever was needed to send her away again, and he wasn't going to let them do that.