There is an unofficial referral system that is known to social workers and medical doctors where the dying are selectively referred. This is not something special just in singapore. Most developed countries have something similar where medical marijuana is allowed in selected groupings. The activities are monitored so that the unscrupulous charcters don't take advantge.I was very surprised to learn from my campmate in the mid-90s that his grandfather was an opium smoker. He got his daily fixes from a flat in the Redhill/BHS area.
Apparently the CNB knows of this den but leaves it alone since most of the clients are very old men who could just kick the bucket anyday. If the den was raided alot of old addicts would just die painfully.
Some of the elderly are brought by their well heeled children and dropped off to be picked up later. Like the yellow card system in the "licences brothel" where the brothel can only be passed to another relative, there are some controls.