Why does everybody simply believe the words of the So'na? They cheated the Federation and stabbed it in the back so why shouldn't the have lied about the medical benefits of the radiation?
You know this really makes no sense? If the S'ona simply wanted to exterminate the Ba'ku, they could've run in, did the deed and been out before the Federation was aware of what happened.
It wouldn't have taken long since all six hundred are in a centralized location, not spread all over the planet.