I don't think realists have to have hidden motives like a desire for fame, but it is not clear to me how they could be motivated to act on what they take the stance-independent moral truths to be unless they wanted to do so in some way. This isn't so much a metaphysical principle as it is a recognition of what motivation is: motivation is whatever psychological processes prompt agents to act on their goals. Maybe others think of motivation differently, but that's what I think of as motivation.