There are only a few ways I can think of for them to even pretend to justify this:

• Defining a feeding tube as a non-medical treatment which may not be refused.
  
• Defining refusing vital medical treatment as suicide, for which the patient may be committed and treated involuntarily.
  
• Adding some sort of due process before allowing the exercise of medical power of attorney to refuse vital treatment.
  
• Granting doctors and nurses a freedom-of-conscience power to ignore patients' wishes in some cases. *shudder*
  

Any of these would of course add to the very long list of powers Congress claims without the states ever having delegated them.

At least there's nowhere higher for this issue to go. If Congress doesn't manage to interfere, that should be it.