If they are violating the rules, then it's reasonable for the government to shut down their casino businesses knowing that the safety of the people there will be at risk. The government of Thailand is actually doing great with how they take action towards the pandemic since they have low covid cases compared to other countries and they are probably just preventing another wave. Aside from their aim to prevent the spreading of the virus, illegal gambling won't do any benefits to the government that's why they are being strict with it. From what I have searched, there's only a limited legal form of gambling in Thailand and maybe that causes people to operate illegally. Their government can work on legalizing gambling/casinos in their country since they can benefit from it but I don't think it's the right time to focus on it right now since it can stimulate public gatherings.