Its just there are some steps missing.
1. You cannot prove physical causation.
2. Therefore there is no physical causation. (A fallacy by the way).
3. So the physical causation hypothesis, in this case ME (or CFS if you want), is wrong. (This relies on step two being right, which it isn't.)
4. That leaves psychosomatic causation as the only viable hypothesis. (Yet another fallacy).
This relies on not one but two fallacious steps in reasoning. Further, its applying a double standard, one which leaves psychiatry out on a limb, in a storm, and the limb has started to creak.
There is no proven universal physical caustion for CFS (or ME if you want, but they usually argue CFS). Where is the proven causal mechanism for ANY psychosomatic illness? Show me the proven mechanisms, step by step, and the definitive papers which demonstrate this!
The whole notion of definitive physical causation is sometimes a smokescreen because there are lots of biomarkers. Show me the biomarkers for psychosomatic illness! There HAVE to be some, at least in most cases, because the outcome is physical, and measurable. So at some step the "mental" causation must transform (induce, trigger, etc.) physical outcomes. So there is NO excuse for no biomarkers. This is further complicated by the huge heterogeneity in most conditions that are defined by symptomology. We really need an ME biomarker to make our point drive home.
Psychosomatic as the only viable hypothesis is an invalid leap. What if its something we cannot measure, do not know to measure, or have failed to measure accurately? What if its based on disease mechanisms still not uncovered or understood? What if its environmental toxins (including mold)? What if its electromagnetic damage? The mechanism behind at least one type of electromagnetic damage has some evidence for it, unlike psychosomatic claims. Heck, what if its the Illuminati, aliens, or an international conspiracy? Why stop the explanations at psychosomatic when the evidence for it is about as good as alien conspiracy?
The problem with psychosomatic claims is simple - lack of evidence, and lack of sound reasoning.