Hi,
I'm not able to reproduce the problem with the 6.1.0
I properly get the message "You must have bought this item in order to post a vote or a comment" instead of the comment / vote form when I access a product which I didn't bought while being logged in on the website.
Could it be that you have some old view overrides in your template which prevent you from having the patches we made ?
Or maybe it could be that you have page caching on your website and thus the users all see the same page ?
Check if you have the folder templates/YOUR_TEMPLATE/com_hikashop/ via FTP.
If you do, try renaming it to something else temporarily in order to deactivate all the overrides for HikaShop. Also, clear the cache of your website, and then try reproducing the issue. If you don't have the issue anymore, then it's probably coming from one of the override files or the cache. You could try to renaming the product subfolder or the vote subfolder to try to narrow down the source. And then the files in them.
If, even with the overrides disabled and the cache cleared, you still have the problem, then please provide a backend access and a frontend access with the issue and the URL of a product page with the issue via our contact form so that we can look at the issue:
www.hikashop.com/support/contact-us.html