Son'a ... why didn't they just tell the UFP that they have a claim on the planet
Because their hold on the entire planet and the rings was just as thin as the Baku. The Federation's legal possession of the area wasn't in dispute, which is why the Sona brought the Federation in to the arrangement.
If the Sona felt that they had a clear and legal claim to the planet, one that the Federation wouldn't contend later, they would have simply moved the Baku out without ceremony, and harvested the rings.
Problem with that would be the Federation did see the region as theirs.