no smoking, by law!

Asking a Singaporean, why there is "by law" label below the No Smoking sign:

Singaporean (S): that means you shouldn't smoke there
me: No Smoking sign means that anyway,.. why there's need of that label?
S: if you don't put "by law" means you shouldn't smoke, but if you are caught, there is no fine, they will just tell you do not smoke and let you go.
me: huh?
S: if there is "by law" label, means they can fine you if you are caught smoking.
me: so if there is no "by law" label,... you can smoke there?
S: you can't,.. but you don't get fined if you smoke there. "By law" means it's a stronger restriction.
me: I do not smoke.