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.
Guardians, such as parents, are granted expansive powers over incapacitated persons and minors. For example, it is assumed that parents would know the wishes of their children and would act in their best interest. I agree. If the people of the states want to overturn the rights of guardians in situations like this, they need to lobby their legislatures and change the laws.