Legally, it’s not compulsory. Socially and culturally, it definitely is. In the US, if you don’t tip 15%, you’re either an asshole, or you’re saying there’s severe issues with the service.
I’m not going justify this culture—I don’t like it either—but that is the way it is.
I’m not going justify this culture—I don’t like it either—but that is the way it is.