Hi,
Its purpose is to redirect to HTTP outside of the checkout so that only the checkout uses HTTPS.
It is to be used in conjunction with the "force SSL on checkout" setting of the HikaShop configuration.
This plugin is disabled by default so if it was active, it means that you must have activated it. As you say, it has no place overriding Joomla on that and that's why it is disabled by default. But some merchants want to be able to have HTTPS only on the checkout so the plugin is there. It doesn't mean that you should use it.
hikashop_config is not available unless you include the HikaShop main helper file. The no SSL outside checkout will load that helper when checking if the current page uses SSL or not.
But in your own code, you should always load the main helper if you need HikaShop functions and you're not sure whether HikaShop is already loaded or not:
if (!defined('DS'))
define('DS', DIRECTORY_SEPARATOR);
if(!include_once(rtrim(JPATH_ADMINISTRATOR,DS).DS.'components'.DS.'com_hikashop'.DS.'helpers'.DS.'helper.php')) return true;