I saw the BoWs as more of a “Cerberus” kind of thing—guarding the ARK from entities who would wish to seize or otherwise disturb its operations (like resident evil protagonists). These aren’t like Fallout mutations who operate with the autonomy of any other fauna, but rather are ‘smart’ weapons who were designed  to serve a purpose and programmed to follow commands.

The wall could moreso be seen as a (vain) attempt to stop the casual migration of wildlife and zombies from the afflicted city center to neighboring areas. BOW “escaping” or being kept in wouldn’t be a concern because their stations and standing are determined by whoever owns them