Per Nimrod Papp:
By default Site Administrators can display content in Sites from other Sites they can administer. This means that you are able to select Web Content Articles in Web Content Display portlets from other Sites if you are a member there.
This functionality can be turned off however, as documented here:
Customers can also use this property in portal-ext.properties:
Unfortunately the connection between this documentation and the Web Content Display portlet is not obvious, so many of our engineers and customers are not aware of this option (as a result we got a negative feedback on a customer ticket: CAS-6759).
Would it be possible to mention this configuration option in our documentation where the Web Content Display portlet is described?: