1. It's also "mainly or exclusively for pedestrians." with a `vehicle=yes` case. There's even a `=destination` case which could simply be `=residential` etc, as is what's used in HK now .  Clearly, your interpretation is not only how it's used. https://wiki.openstreetmap.org/wiki/Tag:highway=pedestrian#Examples 
2. Can you give examples of such routers? The argument doesn't hold when `=pedestrian` can indeed allow vehicles. They should still evaluate any `vehicle=` / `motor_vehicle=` / `*:conditional=` first. 