I've posted on TASER procedure before. Knives are lethal weapons. You don't face a lethal weapon with a LTL device. That's a standard TASER policy.
You could also Youtube the training video "Edged Weapons" or Google "Tueller rule" or "Tueller distance," and learn how it easy it is to kill someone with a knife even when you're twenty-one feet away.
As usual, it all depends on the situation.
If you are alone, facing a man with a knife, you shouldn't even think about using the taser. You are facing potentially lethal force and not only policy but common sense dictates that you employ your firearm.
On the other hand, if you have four or five officers surrounding a man who is not actively threatening them with a knife but simply holding it and refusing to drop it, that's a different story.
You can have officers with their guns trained on the man and at the same time have an officer use a taser on the suspect in an attempt to disarm him. As the OP says, without a lot more information on the exact circumstances of this case, it's impossible to say whether or not the shooting was justified.