Windows Server Backup or wbadmin.exe cannot be used to backup passive databases in a Database Availability Group environment.
WSB does not back up Passive copies. It can only be used to back up Active copies. This is simply a limitation of the WSB plug-in. Also WSB is only designed to backup small Exchange 2010 environments that are not DAG members.
Also – as indicated in the article, you have to back up the entire volume containing both the database/logs.
So essentially if you want to use WSB to take a full backup of *all* databases, you are looking at either moving all active copies to a single DAG member (for the backup), or using WSB locally on each DAG member that hosts active databases.
Please note, you can backup passive mailbox databases using WSB in a DAG environment as explained in the above article:
"If a server hosting the data being backed up is a member of a database availability group (DAG) and hosts both active and passive database copies, you must disable the Microsoft Exchange Replication service VSS writer. If the Microsoft Exchange Replication service VSS writer is enabled, the backup operation will fail."
Why would you want to use windows server backup to backup a DAG member? Possibly your primary backup product is failing and you want to test the VSS service using an alternative product? In this case you may use BETest.exe to test the VSS snapshot service.