Most people understand that we cannot usually infer ought from is. But what religious people like Craig seem to be doing is committing the even worse offense of what one might call the 'ought-is fallacy', where because they think that we need an objective morality in order to keep our barbaric impulses under control, therefore it must exist. And since they also think that only a god can supply such a morality, therefore a god must exist also.
Believers in god have to first establish using empirical evidence that god exists before they can use god in arguments about morality or anything else. You cannot argue for the existence of god on the basis of some property that you arbitrarily assert must exist (for whatever reason) and that could have only come from god. Link.