Why are three of the five base location choices unavailable?

In XCOM: Enemy Unknown, I have to pick the location for my base after the initial tutorial mission. North America and Europe are available, but Asia, South America, and Africa are not.

There's no explanation for why, though. Do I need to play through a game to enable them? Is it a difficulty level thing? How do I build my alien-fighting empire from those locations?

You only get two choices if you are playing the tutorial. However, if you start a new game that skips the tutorial, you can choose from any of the 5 bases. You do not need to beat the game or anything like that first.