It is confusing. 'All' technically means all applicable rather than all except those specified. It depends on the way, but for instance, a cycle and foot path that allows motor vehicle access from land owners but does not allow horses would be left untagged defaulting to all=yes with motor_vehicle=private and horse=no applied. If this way were permanently inaccessible to anyone or only to government/emergency vehicles, the default all=yes should be changed to all=no, which will make the road inaccessible on routing systems and render as such.