ADMINISTER DATABASE TRIGGER privilege
The ADMINISTER DATABASE TRIGGER privilege allows you to create database-level triggers (server error, login, and logout triggers).  It also allows you to log in regardless of errors thrown by a login trigger as a failsafe.  If you inadvertently coded your login trigger to throw an error no matter who was logging in, for example, you need to allow someone to log in to fix the trigger.  

It appears that the trigger is behaving as expected in this case.  You wouldn't in reality create a database link from one database to another using a DBA account.