found the bug:

see this function in the ContentProvider:

public string GetNodesContent(string path, TreePathTypeEnum pathType, string className, string transformationName, string where, string orderBy, int maxRelativeLevel, bool selectOnlyPublished, int checkPermissionsForUserId)
{
DataSet resultDS = TreeProvider.SelectNodes(path, pathType, className, where, orderBy, maxRelativeLevel);

// check permissions
if (checkPermissionsForUserId != 0)
{
TreeSecurityProvider treeSec = new TreeSecurityProvider(TreeProvider);
resultDS = treeSec.FilterDataSetByPermissions(resultDS, NodePermissionsEnum.Read, checkPermissionsForUserId);
}

return Transform(resultDS.GetXml(), transformationName);
}

the selectOnlyPublished isn't passed to the SelectNodes method.