> Because they actually derive their entire income from work done and assets owned in the USA

How many people does this even apply to, compared to the vast number of US citizens abroad (some of whom are not even aware they are US citizens) who owe US taxes on income from local sources? If you really think this makes sense, then tax it, but that isn't how income tax works today for expats who are taxed on all of their income sources (globally) above the exemption.

> don't even really live in Singapore

There are laws in place to establish residency with a reasonable degree of enforcement.