Hi,
I have test it with the latest HikaShop package and the javascript is working.
If you have some javascript errors in your page, it could be possible for a customer to add the product in the cart when the field is empty.
If you use the "add to cart" button in a product listing, where the custom field is not displayed, the restriction could not be test, so the product would be added.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.