I cannot reproduce this issue on any of the test systems running on an https:// protocol. The destination link remains with the original http:// protocol in the URL.
I’ve also tried this by specifying http://www.
The ONLY way I can get the link in the location listing to come up as https is if I specify it as https.
There are ways within WordPress to force SSL URLs using WP hooks and filters. It may be that a non-SLP plugin is changing your URL processing rules and forcing https on ALL content including the JSONP response coming back from the WordPress AJAX processor. Hopefully that is not the case, but it appears that the problem may not be within the Store Locator Plus codebase.