I knew a man who had stomach cancer so had most of his stomach removed and he had to be fed by the feeding tube. There came a time when the cancer returned and he said he didn't want to go through anymore so just remove the feeding tube and let him die.
Now this person was able to tell everyone his wishes, which Terri is not but it does point to a circumstance which is permissable to do.
The point is that those people are dying from the cancer already, while Terri was *not dying* before they yanked her feeding tube.