That's because it wasn't a criminal trial; it was a civil one. Civil trials have different standards of proof and of evidence. Judge Greer is not a bad judge just because this case didn't fit the legal system very well.
Clearly, some law should be enacted to move civil cases involving a life up to the same, higher standards as a criminal trial. But Greer couldn't try the case based on what he wished the law was. He had to try it within the laws he was given.
It's normal for people to want to demonize the judge when a case doesn't turn out favorably. But Judge Greer wasn't the bad guy; Terri's husband is a much more likely suspect.
It was so flagrant, you'd almost have to deduce that Greer and Felos worked out the script together. Greer did everything Felos asked. He stripped Terri's trust fund to pay the lawyers -- to work against her with her own medical therapy fund! (Greer undid a real jury's work with that one.) He ignored guardianship regulations for Michael again and again and again. He refused to allow Terri a Guardian Ad Litem. He fired the only real GAL Terri had because the man tried to represent Terri's interests -- which, of course, is precisely what he is charged to do. (Greer himself took over as GAL, which is like the fox guarding the chicken.) He sealed medical and financial records that would have been embarrassing to Michael's case. His finding of "clear and convincing" evidence that Terri asked to be put to death would have been laughed out of a soap opera. It was transparently rigged. And that was his excuse to kill her.
Even if everything he did to this point was legal (hypothetical! it wasn't!), he ruled ultra vires that she was not to receive food or liquid by mouth. That is NOT allowed by law. It made him a murderer.
Can you starve your dog in a civil trial?