Why can't they make it a federal crime for the medical community to deliberately starve/dehydrate a person to death?
That is illegal in every state, with the follwoing exception - wishes of the patient. You are free to starve yourself to death starting right now. And that is what the court found, that Terri would willingly starve hereself to death, if she was afforded an opportunity to decide.
The mumbo-jumbo legalistic handwaving notwithstanding, this is an abhorrent result, because I doubt anybody would wish to starve to death.