That's a stolen power -- they have it only by stealing it. Any person has the power to murder another. Under law, NO person -- Judge nor King -- has that power, rightfully, to use against an innocent.
No, they have it because of their status as the legal guardian. There's dark conspiracy here, just the way the laws are currently written. It is assumed that the legal guardian would know the wishes of the incapacitated person, and the law gives them the power to carry out those wishes.
Have the law changed if you don't like it -- that's your power.