Further to the earlier posting, you can check out the current NCPG website. Other than the well-known Voluntary and Family exclusions, there is also a Third Party exclusion. The following is from the NCPG site:
Third-Party Visit Limit
If you are a frequent local casino patron who is deemed to be financially vulnerable, a limit may be placed on your visits to the casino under Third-Party Visit Limit.
There is no need to apply for Third-Party Visit Limit. To contest or revoke a Third-Party Visit Limit, call NCPG at 6354-8154.
You will be required to submit supporting documents to prove your case and will be given reasonable opportunity to object to the proposed visit limit. The final decision to impose a Third-Party Visit Limit lies with NCPG.
http://www.ncpg.org.sg/draw-the-line/visit-limit-measures.html
3RD PARTY EXCLUSION
The Third-Party Exclusion will automatically exclude undischarged bankrupts, and people receiving financial aid from the Government of Singapore, from entering the casinos in Singapore. There is no need to apply for this exclusion.
http://www.ncpg.org.sg/draw-the-line/casino-exclusion-measures.html
The persons who received the letter is probably facing some financial difficulty. It is not stated in NCPG site how a person is deemed to be financially vulnerable. The Income Tax Act prohibits IRAS from giving your tax returns to NCPG. A more likely reason would be a database matching of individuals who applied to the government for some kind of financial assistance or is facing bankruptcy proceedings.
Other than that, another possible reason is that someone in their family objects to their gambling habit and applied for a family exclusion with NCPG.
Given your financial status and the fact that no one in your family objects to your habit, it is unlikely you will be receiving a letter from NCPG anytime soon.
As i said, I am not aware of any statutory requirement to attend such sessions. I might of course be mistaken which is why I said the safe thing to do would be to ask the officer under which section of the Casino Control Act that they are invoking. Unless it is written in the act, they have no power to compel you to attend the session.
Important to note that the letter/reminder will be written in such a way so as to give the impression you must attend. If it is really required by law, the Civil Service is required to quote the relevant section to the respondent so that it is clear that it is not an option but a mandatory requirement under the law.