Originally Posted by HDQDD

I suspect because SIN is a city/state with flights only to other countries this is more of a border security issue than anything. If you buy a ticket and go through security and presumably border control (so you’re now in a “transit zone”) and then intentionally don’t board you could face penalties like those mentioned in the article. Also some terminals are designed only for departing pax (on the departure level), so if you tried to stay you would have to find someone to help you get into the customs areas to re-enter.

This policy wouldn't apply to any US airports and only Intl ones where one would go through passport control enroute to the sterile area.