almost all secular people do in fact believe in objective morality to some degree, though you can occasionally find a pure moral relativist or legalist

remember that the holocaust, the great leap forward, gulag, the imprisonment of galileo, daily human sacrifice to the aztec sun god, and witch-burnings in the spanish inquisition were not just legal but legally required

therefore, anyone who condemns one of them is appealing to a moral standard they consider superior to, at least, the legal systems of the countries in question at the time