This is because the word "Muslim" contains in its Arabic meaning its own definition.
You see, the word "Muslim" in Arabic has two parts: the "Mu" prefix and the triliteral root that forms the word "Islam." That root word, "Islam," is a verbal noun that means "submission." When an "Mu" prefix is attached to such a root in Arabic, the resulting noun means "a person who does the thing that root word denotes."
Therefore, with "Islam" being a verbal noun meaning submission, "Muslim" therefore means "one who submits." Submits to what? To Allah's will, which is shariah. Islamic Law. Thus, anyone who presents as a Muslim is by definition shariah-adherent, because that's what the word itself actually means. If someone claims to be a Muslim, or converts to Islam, or was born into Islam but does not apostatize or separate from it, then it is reasonable to conclude that such a person is shariah-compliant-at a minimum, tacitly-unless and until told otherwise. And the converse must also be true: one who does not submit to shariah, one who does not adhere to shariah, does not meet the linguistic definition of "Muslim."
It is this commandment to Islamic supremacism that is most problematic for non-Muslims and responsible for much of the debate about what exactly "being Muslim" means. But if we realize that the answer lies in the etymology of the Arabic word "Muslim" itself, then it will be understood that unless and until that identity as "one who submits" is abjured by the individual in question, the person is accorded full credit for living a shariah-adherent life.
Clare M. Lopez is the Vice President for Research & Analysis at the Center for Security Policy. Read more at Family Security Matters.