That's a very good question, and there is actually a clear, rational answer to it. The difference betweem the two is the cause of death.
If someone sustains catastrophic injuries in a car accident and is removed from a ventilator, then his death has been caused by his catastrophic injuries. In other words, the fact that the body is unable to sustain itself without the aid of these machines is an indication that death is a clear consequence of the injuries. Keep in mind that I am talking here about someone whose long-term prognosis is not good . . . my statement would not apply to a car accident victim who is on a ventilator on a temporary basis while undergoing surgery that has a reasonable chance of successfully treating the injuries sustained in the accident.
If this person does not require a respirator but needs a feeding tube to stay alive, then removing the feeding tube changes the manner of his death -- and his death is caused by starvation, not by his injuries. In other words, starving a bedridden patient to death is no different than shooting him, because the subsequent action replaces the initial injury as the "proximate" cause of death.
Well, I remember the era of the iron lung. Would that be the same? Also, if a person suffers an injury which cannot be healed and cannot eat, doesn't the injury then cause the death because the body would be unable to sustain itelf without the aid of an artificial device?