As with most WT related questions, the answer to the topic line question is, "Because the GB says so."
But why the distinction? I don't know. Maybe it has something to do with the GB seeing the agent of death as the criminal in the case of a deadly assault but as Jehovah in the case of any other kind of death. In other words, the GB would see self-defense as okay to stop a criminal but taking blood as a rebellion against Jehovah's right to decide whether or not you will live or die of any other sort of cause.
Then again, the distinction may be based on nothing more significant than the mood Joseph Rutherford or Fred Franz were in the day the particular WT articles were published.
Somehow the GB considers it one of the highest proofs of loyalty and endurance if you choose to die or let a loved one die rather than take a blood transfusion. In the twisted WT works salvation system, what better way to guarantee resurrection than to die because you obeyed the faithful and discreet slave to the point of death by refusing blood? (Remember the notorious cover of deceased children who "put God first" by dying because their parents refused to let them take blood?)
Of course, if you do pass the "no blood" loyalty test with flying colors and survive the health crisis anyway, you can lose all your WT credits the next month by slacking off in field service or wearing clothing the elders don't approve of. In the GB's view, you would have been better off dead.