The prop governor does not know what is turning the engine, it only knows that the revs are dropping so it fines the pitch to keep the revs constant. By pulling the pitch lever you reduce the rpm demand and feather the prop but it can never fully feather because the only source of rpm is the windmilling prop.
A fully feathering prop as fitted to bigger twins has a special oil pressure reservoir to drive the prop to the fully feathered zero rpm position.