For better or worse, the community voted that the separate value (highway=busway) should be used for BRT corridors as they are distinct from service roads (and BRT corridors don't meet the definition of highway=service). See the proposal for details of why this distinction was made: https://wiki.openstreetmap.org/wiki/Proposal:Tag:highway%3Dbusway

Bus station access roads were specifically excluded from this (when they are not part of a BRT corridor) as they can meet the definition of a service road (access to a property).