Why didn't the Son'a settle somewhere else in the intervening years? Don't they have an equal right to the planet? etc.)
Plus, you know, it's a planet. Kinda big. Assuming they don't use jet airliners to get around (and they have the same attitude that the Amish do, that the level of technology in about 1750 was all that was ever needed) then they could exile the Son'a to the other side of the planet and that would be just fine.
In fact, Starfleet could also have put a research team anywhere on the planet to study the radiation for years, why the rush?
Because the film was written badly, thrown together as cheaply as possible for bad reasons - sigh.