Question about the dag backup

Hi,

I have a dag with two nodes and I use wsb to backup it. I backup the standby mailbox server (node2) by WSB successfully.The database logs are truncated on the node2.Now what I'm confused is why the database logs are truncated on the active mailbox server (node1) too? In my options, Logs should be replicated from active server to the standby server,then how it happens?