Could being the operative word. They never claimed that they were.
YOU want to give the guardians powers they never had:
The Department of Public Welfare objected, stating that Hockenberry was neither terminally ill nor permanently unconscious and never appointed a third party with the power to refuse healthcare necessary to the preservation of his life.
However IF they had been granted medical power of attorney by the state, then they could have denied the use of a ventilator to prolong or save his life, just the same as anyone else who doesn't want such measures used on them.