In Austria (and maybe some other parts of Europe, not sure), not only that free refills are unheard of, you even get charged for ketchup sauce at McDonalds. (The minimal allowed food quality is higher, though, so there might be economic reasons on top of, I believe, primarily societal reasons.)
It is charged somewhere around 40 to 50 Eurocent per 25ml.
A customer shot employees at a McDonald's because the customer wasn't allowed to dine in the restaurant: https://www.latimes.com/world-nation/story/2020-05-07/police...
But people were pissed. It was only a small portion of customers who even asked for more than the include packages, but they were hot about it.
When I first started, we didn't even give free refills of soda or coffee. And I remember, we had this group of senior citizens who came in every morning as a group to just drink coffee and BS. Well, the tax rate in CA changed one day, bumping the price of a cup of joe up a penny. You'd think we had stolen their Social Security checks. They were outraged. Trust me, you do not want a pissed off senior citizen yelling at you when you're 16.
Soda refills are almost free, the cup costs more than the soda, so besides avoiding an obesity epidemic, the customer satisfaction of giving free refills is probably worth it. Also it probably costs more to pay someone $15/hr to just fill sodas, when you can get the customers to self-service for free.