Migration failed with source plugin exception: SQLSTATE[42000]: Syntax error or access violation

When I try migrating my content from one MySQL into my Drupal site using drush, I'm getting the following error:

Migration failed with source plugin exception: SQLSTATE[42000]: Syntax error or access violation: 1142 SELECT command denied to user 'isps_wp'@'localhost' for table 'migrate_map_isps'                                                                                [error]

When I give the source database's user all permissions to my Drupal database, the migration works fine. But it seems wrong to me that I would have to give my source database's user permissions to my Drupal db to make this work.

I'm pasting my migrate class below. Are there some obvious things wrong here?

// Migration class.
class IspsMigration extends Migration {
  public function __construct() {
    parent::__construct(MigrateGroup::getInstance('ISPS'));
    $this->description = 'ISPS migration';
    // Use a SQL database as source.
    $query = Database::getConnection('default', 'for_migration')
             ->select('wp_posts', 'wp')
             ->fields('wp', array('id', 'post_title', 'post_content', 'post_type'))
             ->condition('post_type', 'page', '=');
    $this->source = new MigrateSourceSQL($query);
    // We migrate into "article" nodes.
    $this->destination = new MigrateDestinationNode('article');

    // We instantiate the MigrateMap
    $this->map = new MigrateSQLMap($this->machineName,
        array(
          'id' => array('type' => 'int',
                        'unsigned' => TRUE,
                        'not null' => TRUE,
                         )
        ),
        MigrateDestinationNode::getKeySchema()
      );

    // Finally we add simple field mappings 
    $this->addFieldMapping('title', 'post_title');
    $this->addFieldMapping('body', 'post_content');
  }
}