What you see when you disable JavaScript is likely to be the Search Engine Optimization (SEO) code which is included on the web page within <noscript> tags and is displayed if JavaScript is disabled in the browser (and the true Juicebox gallery cannot be run).
If you want to protect a gallery from the general public, then it is perhaps also likely that you will not want search engines indexing your gallery page's content so I would recommend that you remove the SEO code from your web page when using Juicebox's built-in Password Protection. (When creating a gallery with JuiceboxBuilder-Pro, deselect the 'Add SEO Content' checkbox from the 'Customize -> Sharing' section.)
As you point out, an alternative method of securing a gallery would be to password protect the gallery's directory on the web server using .htaccess and .htpasswd files. Instructions can be found here. You might also be able to password-protect a directory on your web server from within your web hosting account's online Control Panel (without having to manually set up .htaccess and .htpasswd files.
That would be nice to see it in the next version ;)
Please feel free to post ideas for future versions in the Feature Requests forum thread.
This keeps all the ideas together and ensures that they are not overlooked by the developers.
Thank you.