When oauth is used (or at least when calls to pulp are made by katello), katello performs all authentication/authorization checks, therefore pulp shouldn't expect that the user passed in "pulp-user" header will exist in pulp's user database; this information is passed in mostly for logging purposes.
Could whoever picks up this issue contact dmitri / witlessb on #pulp when it's fixed?
Talked to Dmitri: Katello is now creating a Pulp user with the appropriate permissions. Closing as not a bug