Hi,
1. I'm not able to reproduce the problem. When I try this, I'm not able to see the listing of products of that category, instead, I get the categories/products from the main category of the shop. That's because when the category is loaded there is a check on the ACL and if it doesn't load the category in the database, it uses the main category instead of throwing an error.
2. Being able to display a list of products from different categories of your category tree with HikaShop is usually done by creating an unpublished category, assigning the products you want in that listing to that category (as a second category) and then creating a menu item listing with that category selected.
For example, if you want to have a list of "featured products".
So being able to visit an unpublished category page is something that is exepected.
3. What we could improve though is to throw a 404 error on the access of the page if the category is not the one selected in the main category setting of the menu item if the category is unpublished or with ACLs not granted for the current user. I believe it would "fix the bug" for both points and I think it would be much cleaner.
I've added tha patch for that on our end.
You can download the install package on our website and install it on yours and it should be like you want now.