Illegal. Not authorized by law; illicit; unlawful; contrary to law. (Black's Law Dictionary). Illegal. Prohibited by law. Prohibited by official rules. (The American Heritage Dictionary) Now what part don't we understand.
Islam does not allow people to choose their religion of choice. Because, Islamic Allah (God) accept only Islam as the religion of all mankind. Islam never recognizes or respects any other religions, period. Islam never tolerates any other religion but Islam. What a peaceful religion. Ha!