The whole case was framed on the ludicrous fiction that Terri made the decision, as is her right (to refuse treatment). The feeding tube was added by law to life-prolonging devices AFTER Terri's injury, so she could not possibly have given informed consent.
Does not matter.
The guardian has the right to make any and all medical decisions concerning treatment, including withholding treatment on the advice of doctors. We don't expect people to have specifically prepared for every possible situation - if we did, doctors could just follow those instructions and there would be no need for a guardian. Sometimes the guardian is left without explicit instructions, and he has to choose what he thinks she would want. Such is the nature of guardianship.
It is his legal right and responsibility, and moral obligation to make those difficult decisions in what he and he alone determines to be in the patient's best interest.
Just because you don't personally like the decision he made does not mean we should go crying to the State to save us from having to make those difficult decisions.
By law, if Michael made the decision, it was murder. Which, of course, is why Michael and his lawyers always vehemently deny that it was his decision.