No they understand just fine. Here's a quote from an ethics book that gets at the same issue:

The extreme sexism at the heart of honor killings is but one of many examples that raise doubts about cultural relativism. After all, societies are sometimes based on principles of slavery, of warlike aggression, of religious bigotry or ethnic oppression. Cultural relativism would turn these core ideals into iron-clad moral duties, making cooperation with slavery, sexism, and racism the moral duty of all citizens of those societies. The iconoclast—the person deeply opposed to conventional wisdom—would, by definition, always be morally mistaken. This has struck many people as seriously implausible.

Russ Shafer-Landau - The fundamentals of ethics p.293 ("Some Implications of Ethical Subjectivism and Cultural Relativism")