Currently, a task assignee also can abandon a workflow.
For instance, if joe starts a workflow and assigns the first task to jack, joe and jack can abandon the workflow.
Only joe (the initiator) and an administrator should be able to.
Note that we have the same behavior in Web UI because we are simply relying on the runningWorkflows enricher (at least one => can abandon).
In this case, jack, as an assignee, has a runningWorkflow on fetching the document.
In JSF, we are relying on an action filter: isAdministrator || isInitiator.