I just looked back over the Hoenn Pokedex, and there are a ton of Pokemon that could have suited either team (Well, mostly Aqua) that weren't used. I guess it was an issue of uniformity, perhaps? Maybe a justification for it would be that the two teams have strict guidelines for what Pokemon may and may not be used. Just theorizing here, of course.
They would barrier off a certain area of the coastline and fill the boxes in with the ashes which makes new land! Therefore Singapore would be a wonderful place to recruit for team magma =P.
That's pretty interesting, actually. Though I'd have to say, as a general rule, assuming that the general population actually cares, that any overpopulated country or country that seeks expansion might be a good place to recruit. This is assuming anyone cares, because there are a lot of people who won't have a second thought about things like the environment for as long as they live.
Those people obviously aren't going to be joining a team that is well...environment oriented, I guess.