Root is never asked for the existing password when changing a password because as root - linux does not question you. What root says, linux does. Which is why having root is both incredibly dangerous, and powerful.

Sudo gainroot is configured to not prompt the user for a password to, naturally, "gain root". Normally, sudo WOULD ask for a password.. however this was intentionally configured not to both because there is no passwords on the device by default... and it's simply easier.