According to the opposition open letter, Plan S includes "A prohibition on publishing in either subscription or “hybrid” (i.e. partially open access) journals,"

According to the opposition open letter, Plan S includes "A prohibition on publishing in either subscription or “hybrid” (i.e. partially open access) journals," I don't see why that should be a requirement. So long as papers are made available for free, why should it matter if they are also available in a paid journal?