Here's my theory: The parents see a career for themselves on the fundie inspirational talk circuit. And/or a book deal.
First of all, it comes out that the split between husband and in-laws--formerly devoted and united in their mission to care for Terri--came about in an argument over the malpractice settlement money. According to Michael, the father expected Michael to split the loss of consortium award.
Second, thanks to the great research on this site, we know that there is oodles of fundie money backing the Schindlers' fight. (And, as previously noted, no offers to pay for Terri's care--just money to fight the public court battle.) With thousands of people in similar situations, does anyone think this is all about Terri for Scaife, Jeb et al.? This is a hand-picked PR extravaganza. This is an investment.
Then I think about that hyper-Christian general in the Iraq war (is that his rank?), the guy who became infamous for his racist Crusader remarks. He's big on the fundie circuit, giving inspirational talks about how God personally intervenes to save the lives of soldiers he prays for. I could be wrong, but I'd be surprised if there weren't some nice honoraria in it for him.
Now just imagine what gold these people, the Schindlers, would be on the circuit, telling their story, how they brought Congress to heel, made a miracle, with nothing but prayer and cable news. (And frankly, my money is on them winning at the Supreme Court level. I don't trust this court, and there is way too much invested for the fundies to be denied their happy ending.) What a perfect opening act for the crusade against abortion, and god knows what else these scary people have their eyes on.
Which is not to say, because I know this will rub everyone the wrong way, that the parents haven't convinced themselves they are doing the right thing. That they don't love their daughter. But I do think they've been seduced--by the promise of money or the fame or both--by a very cynical bunch of puppetmasters. That their job is (and will be) to provide the theater, with which the fundies will rally their Christian soldiers.