I think the reaction from your American hosts was either unusual, or they privately disagreed with your assessment of the server's rudeness. Maybe that is a restaurant they wanted to return to later and didn't want to burn bridges.

I don't hesitate to refuse a tip if the server is truly rude, but I'll also not return to that restaurant for a long time. This has only happened a handful of times in my life. If the service is neutral then I'll leave a neutral tip, usually 15% unless the total is low and rounds up to a nice whole number.