Unlike a country, though, the island would not have been able to support itself economically once the plantation work had ended.

It's a bit like a country supporting a whaling station on an island[0] and then later evacuating the island rather than continuing to pour resources in to maintain the presence of the now-unemployed workers.

For additional context, I should point out that the UK government did worse things to an island much closer to home[1] (albeit an island without any human residents).

[0] https://en.wikipedia.org/wiki/Grytviken

[1] https://en.wikipedia.org/wiki/Gruinard_Island