I think its due to Singapore's history with drug use, opium additions which got the country kicked out of Malaysia.
That plus both countries had very different ideas about how the country should be run.
Singaporean citizens and PRs can be (and are) randomly drug tested when coming back to Singapore and if found guilty can be fined and/or receive a prison sentence. Yes, you can go to prison for flying to Thailand and smoking legal marijuana.
As for the death penalty, my guess is Singapore will eventually stop mandatory death penalties. Global pressure and stigma will eventually be too much. Even their neighbor, Malaysia, has stopped all executions since 2018.
However, if they end executions, they'll make sure to spin it as their idea and will give no hint that they bowed to international pressure or opinion.
So what legal chemicals a person imbibes in another jurisdiction, can, if detected in SG, lead to a prison sentence in SG. Wow. In some way, that's like a lack of respect from SG toward the laws of the place where it's legal. Law is not just about punishment, it's also about protection.
So basically SG is now like a much shinier, better run Changi prison, where the prisoners are insanely productive in high-impact industries. I guess it makes sense, even the airport is called Changi.
This does suck tho, as it is a very nice airport, and everything looks very beautiful.
I did not expect this thread to enlighten me as to just how very fucked up Singapore is. Very interesting.
But even such history doesn't mean a place can't move with the times. Also, I don't think you can blame capitol punishment on this.