Chelsea was 27 at the time. She was an adult, taking part in a political campaign. She had every right to do so, and I'm sure the did it of her own free will, because she probably loves her mother and father, and wants success for them, as they envision it.
Meghan McCain did the same thing for her father. She is over 21, and is free to do so. I noticed that Bridget did NOT participate overly much, but she's a teenager, and I'm sure the McCains didn't want to subject her to the cruelty of the MSM.