Um, because whether it's arterial or venous constitutes a material difference in diagnosis, effect, symptoms, treatment, post mortem findings, you know all those important things in both clinical and legal settings. And for the jury to fully understand for the purposes of deliberation.
Unfortunately it's really not good enough for Dewi to just go "air - baby collapsed - baby died" when full life sentences are based on it.
Frankly, your complete dismissal and disregard for the medical evidence as a whole is frightening.
One might assume if you went to the doctor with symptoms worrying you, and the doctor said I'm not sure, but I think it's a full moon tonight, you'd be happy with that. And if you were arrested, despite being innocent, you'd just roll over and say "it's a fair cop guv". And if that's the standard you believe is fair and just in this day and age, I really don't know what to say. Other than it's a sad indictment of the state of the world indeed.