"So why have he and his mother, former first lady Nancy Reagan, made stem cell research their cause celeb? "
Two words: Patti Davis.
The compulsion of former President Reagan's children to disrespect him while he was in office, and after he left office, is only surpassed by what they are planning to do now that he has passed away.
Its "mommie dearest" without the rationale.
I'm not sure I understand your post. Are you suggesting the kids are using their mom? Have they exploited her love for her husband to manipulate her on this issue and try to remake the "Reagan" image in opposition to their father's positions?
I know very little of what Ron P. & Patti have been up to in recent years. Michael's been a self-promoting, exploitative mouthpiece and Mo, who seemed to be the most decent and genuine of the tribe, unfortunately passed on.