Oppressive regimes are ineffective, people will tell you. You make everybody unhappy, they don't work as hard, and you incur extra costs to pay for all those soldiers. Wouldn't it be better to just be nice to your people and see them be nice in return, to pay them good wages and get good, efficient work out of them? Of course it would be! And it is. As long as you can keep people satisfied and voting for you, it is far more efficient to run a free state.
If you play for long enough, the honeymoon ends. Your population keeps growing, but there are no more jobs because the island only has finite natural resources. Eventually, there is simply no way you can provide everyone a good standard of living. There will be many unemployed and needy, who will be miserable due to lack of housing, jobs and entertainment before they emigrate. But they do not do so immediately! Instead, they stay for a while in your island, casting votes against you in elections, and clogging up the facilities, denying service to the working population that would otherwise be content. Now the working people are also angered, since they cannot get food, entertainment, healthcare, and religion. They are also angered because what they see is against their political beliefs. They too, start voting against you.
Set "love it or leave it"? And what if that isn't enough?
This is when you might scratch your head and say: "That's it! I've had enough from you, Tropicans!" and let loose the dogs of martial law.
The biggest vice of military dictatorship now becomes its biggest strength - it drives people off your island and that is exactly what you want in some cases. People might be miserable, but they cannot vote you out. At the same time, one of the biggest difficulties a dictatorship faces - rebels, is going to be largely mitigated by free emigration. Most people would much rather leave the island than fight you, and the few zealots will be easily delt with by your army.
For extending the life of my island beyond mature age, here's a salute to my soldiers and generals!