The portal systems may be setup by department (for example) and the bulletin only applies to specific departments
Bulletin Audience permissions were updated in v10.3 to allow Stakeholders to be defined so that you can target bulletin messages based on the Self Service Portal Logged In User's Organization or Location.
Bulletin Audience needs to also include Portal Systems and also needs to have more granularity to filter based on SSP Portal Role/Subscriber Group/Stakeholder Roles as well.