I cannot fault the mappers for using that combination. These paths are designated by signage and by design.
Yesterday I reached out to openstreetmap.de issue tracker MTB "Single-Trails" erscheinen als Radwege · Issue #61 · fossgis/openstreetmap.de · GitHub because their map shows these same as cycleways. From the response I gather, looking at three tags enough for conclusive classification.
Few weeks ago I also did reach out to graphhopper, osrm and valhalla, if they’d want to honour mtb:scale. GraphHopper now does, checked right now. The others no reply. Will have to wait a bit longer. UPDATE: OSRM does not route bicycles over mtb:scale tagged ways.
PS: I like that idea
piste:type=mtb* – could be used on its own or in combination with path. @yvecai create openmtb.map 