I think it's more that drug limits and their corresponding punishments are clearly laid out in the law [1].

Singapore's police force and public prosecutor also practise a fair amount of prosecutorial discretion when it comes to charges [2], depending on how people plead their case before it goes to trial.

[1] https://sso.agc.gov.sg/Act/MDA1973?ProvIds=Sc2-#Sc2-

[2] https://singaporelegaladvice.com/law-articles/prosecutorial-...