Is it permissible to salute karate players by bowing, knowing that some of them may be disbelievers?

It is not permissible to bow when greeting anyone, whether in sports or other contexts, because the Prophet (peace and blessings be upon him) forbade it. The Prophet (peace and blessings be upon him) was asked: "Should a man among us, when he meets his brother or friend, bow to him?" He said: "No." Moreover, bowing is a greeting of worship that is due to Allah alone.