Minister Tan Chuan Jin justified legalising online gambling, saying that it is a growing activity and anyway, cannot be stopped entirely. Thus it would make sense to bring a part of this dark activity into the light so that it provides people with a guilt-free and legal avenue to engage in this erstwhile illegal activity. This decriminalising of online gambling in Singapore, with certain controls and conditions, will wipe out a criminal activity in one fell swoop. That's the main argument anyway, not to mention that it will increase government revenue from this once illicit activity. Minister Tan went into quite pedantic detail in an interview regarding this matter with Channel NewsAsia. I will not repeat the points, for by Minister Tan, and against by the Worker's Party and the National Council of Churches.
I would just like to add that the government, by the same argument and logic, should legalise the consumption of marijuana (cannabis) in Singapore. It is underground right now, and attracts draconian penalties when caught. In spite of all efforts to eliminate the trafficking of this drug and its use (if no one uses it, no one will traffic it, right?) remains unabated - the same as online gambling. After all, some countries have decriminalise the possession of the drug, including Australia, Austria and the US (in some States) as have some countries online gambling. But I suspect that the Singapore government will not do so anytime soon (or ever), never mind that the logic behind the easing of restriction on online gambling is exactly the same for consumption of marijuana.
I would suggest that the logic and argument that has been brought to bear on justifying the legalisation of online gambling has been quite selective and convinces no one except those for which it is music to the ears.