Because for all of their sophisticated rhetoric, their argument is still based on personal opinion and emotion. They have an "idea" of which way to die is "natural" (via starvation, as opposed to injection or choking on food). They then project this personal feeling onto the debate. From a rational perspective, the method of death is irrelevant, especially if Terri was really PVS. But the "feelings" they have about it matters.
That's why they don't want to discussion the legal implications of this case. Legal operations are rational, not emotional, and gives them no room to wiggle in their emotions. For example, legally speaking, all of the "facts" cited about this case were determined by one man, Judge Greer. I even had a lawyer come on here a few days ago and (after arguing it for a while) admit that it was true. Once Greer decided the "facts" no other court EVER looked at them. Appeals courts look at the way the law was applied, not at whether or not the trial judge decided the facts correctly. So long as the process the judge used to determine the "facts" follows the law, the actual accuracy of the facts doesn't really matter (there was a big stink about this when VA [I think] changed it's death penalty law a while back. The law was changed to limit the time after which someone could challenge certain facts in a death penalty case. What it meant was that, even if you had incontrovertable evidence of innocence, after a certain period of time the court could NOT rehear certain facts. The justification was that it sped up the application of the death penalty, and if the new evidence was that compelling, the governor would pardon the person). So the argument that 19 judges other than Greer looked at the case is sheer sophistry, as not a single other judge ascertained the accuracy of Greer's "facts." Several FR lawyers (including, if I remember correctly, Congressman Billybob) said that the Schindlers really lost their case within the first years of the case, because their attorney didn't prevent the finding of the "facts," thereby making the rest of the case a foregone conclusion (since appeals courts would not revisit the facts).
Add to this my point in previous posts on this thread, and you have a very scary scenario. A single judge, based on the facts that only he determines, can decide to withhold all food and water from a person, in any form. It's no wonder they refuse to discuss this part of it...