Again, it's baffling why the Son'a didn't just tell Starfleet the truth and then have Starfleet let the Son'a come in and do ALL the removal work themselves. Forget the observation outpost, etc.
Because the Federation wouldn't have allowed that to happen, that's the whole point.
They found a somewhat corrupt Admiral in Dougherty, schmoozed him and lied to him. Dougherty was willing to destroy the Enterprise to make sure they didn't tell the truth to the Federation council. And Picard told Dougherty that he was certain that the Court Martial would reveal the truth to the public. The Son'a didn't tell the Federation the truth, and Dougherty didn't tell Starfleet or the Enterprise the truth, because the Federation had the same opinion as Picard on the matter.
But doesn't that defeat the whole "it isn't a Federation planet" argument? Either it is or isn't a Federation planet, people can't have it both ways.