JBossTS is not an application server, it's a transaction manager. It's not responsible for enlisting and delisting resources. That's the application server's job. At the JTA API level, suspend/resume is purely about the relationship between the Thread and the Transaction. That's distinct from the relationship between the resources and the Transaction.
"...The *application server* is responsible for ensuring that the resources in use by the application are properly delisted from the suspended transaction. A resource delist operation [by the application server upon the transaction manager] triggers the Transaction Manager to inform the resource manager to disassociate the transaction from the specified resource object (XAResource.end(TMSUSPEND)). ... When the application’s transaction context is resumed, the *application server* ensures that the resource in use by the application is again enlisted with the transaction."
In practice it's the EJB container and JCA that call enlist/delist and deal with the fun corner cases that arise when you e.g. keep a JDBC Connection open across transaction context boundaries.