support multiple content types

As of simplenews_roles_nodeapi you only support $node-type=='simplenews'.
Simplenews allows us to enable multiple newsletter issue content types.
See corresponding code from simplenews.module

  // Operate only on node types set in 'simplenews_content_types' variable.
  if (!in_array($node->type, variable_get('simplenews_content_types', array('simplenews')))) {
    return;
  }