To make use of Seam managed transactions, you need to use TransactionalSeamPhaseListener in place of SeamPhaseListener.

<lifecycle>
     <phase-listener>
        org.jboss.seam.jsf.TransactionalSeamPhaseListener
    </phase-listener>
</lifecycle>

Seam transaction management is useful even if you're using EJB 3.0 container-managed persistence contexts. But it is especially useful if you use Seam outside a Java EE 5 environment, or in any other case where you would use a Seam-managed persistence context.