Capital punishment is meted out to persons who possess or have reasonable knowledge of someone who possesses or deals in drugs. Do I agree with it? No. But it's very far from your claim. There is due process of law.
>>All members of Jehovas Witnesses were imprisioned in 2004.
The reason for this is because Singapore has a conscripted army. All male citizens are required to spend 2+ years of their lives in service of their country. They are liable to be called up for service any time up till they are 40, or 50 in the case of officers. Jehovah's Witnesses renounce bearing arms and thus their religion is in direct conflict with statute. They are not "imprisoned" in the sense of "rehabilitation" or "punishment". They are confined, so yes there are restrictions on their movement, and are given duties such as gardening, cooking etc. They are free men after they have performed the equivalent length of time as a regular citizen in the armed forces.
Edit: typos