Don't know of any country that forbid arbitrary discounts handed out by the owner or someone the owner gave rights to make such decisions.
(Systemizing such discounts however is definitely an "out", like menus and/or rulebooks stating that people like this or that get a discount.)
Most common reason for arbitrary discounts in my experience is that the person in question is perceived as socially pleasant/improving the general mood.
For people like that, receiving some discounts here and there really aint that strange.
(Kinda like how you might decide to go somewhere further away or more expensive despite tasting the same if the employees in that place are more welcoming/improves your mood.)