Who is "they"? A restaurant is not a person. The only person in a position to do something is the manager who couldn't read your mind.
If I were a manager, I wouldn't need to be able to read my customer's mind to feel that I should proactively address the issue. If a customer of mine had hot soup spilled on them, I would feel compelled to address it, even if the customer didn't make a scene about it. I worked retail for a long time. I know what it's like to deal with asshole customers. The customers I always wanted to help the most were those that were polite, not the people that complain to try and get free stuff.
In this situation, I wasn't going to make a scene over $15. But that doesn't mean I can't also feel that the restaurant failed to address it, regardless of how I reacted.