Baku is a planet in Federation space but it is not a member of the Federation. This is probably fairly common, after all the Feds don't force anybody to join their club.
I think the main point of the posters who think that Picard made an error is that there is a resource of enormous value which should be shared with everybody. I agree, it is not okay than only a few hundred people should gain from a "fountain of youth". But it is neither okay to just steal it from them as the Sona and Federation tried to.
And it is certainly not OK to judge the way of life of any of those numerous lifeforms out there. That's just Prime Directive 101. The Klingons are bloodthirsty predators and we certainly defend ourselves against them but we don't force our ethics down their throat. Biological fundamentals matter, it is very unlikely that such a predator creature would ever join the UFP. Thankfully even the Klingons have a hunch about the Prime Directive, the understand that there aren't just noble warriors like themselves but also coward weaklings out there and while they claim that today is a good day to die they are not that eager to do total war.
Same applies for a bunch of hippies. Not our job to judge them and they definitely don't harm us. But making an alliance with a bunch of "petty thugs" that stab you in the back at the first chance they get just in order to gain an ally in the Dominion War? That's just suicidally idiotic.
I don't mind criticism of the movie, its script is certainly not perfect. But denying what happened on the screen and pretending that fair is foul and foul is fair, that the hippies are wicked resource stealers who don't wanna share (ehm, nobody asked them) whereas the Sona are decent folks is simply factually wrong.
The movie as it stands is a simple morality tale and as FKnight has pointed out there are no options anyway, the Feds have to obey the Prime Directive. And before a big discussion about the Prime Directive starts, I like to watch and talk about Trek. The real Trek, not a hypothetical Mirror Universe Trek where the Sona are our lovely new allies and the Prime Directive doesn't exist.
1. The Prime Directive actually goes against your argument here. If you're invoking the PD then the Son'a should be able to handle the Baku themselves
2. As I and others have repeatedly pointed out, there is ZERO on-screen evidence that the Son'a were going to betray the Federation. This keeps getting brought up, and let's just resolve it right here: what evidence from the movie do you have on this? The Son'a spent MONTHS following the Federation's lead on this, going along with a plan that they didn't even like, just to be cooperative. They ONLY turned against the Federation after Picard sabotaged the plan. Enough with the "the son'a were going to betray them!" nonsense. It's the SON'A WHO WERE BETRAYED!
3. The Baku had been living on that planet for three centuries, never telling anyone else what they'd found. The perfectly logical conclusion from this is that they didn't care at all to bring this revolutionary resource to the rest of the galaxy, therefore it makes perfect sense to conclude that they are utterly selfish and self-centered.
You claim that the Sona had "zero intentions" to betray the Federation which is simply wrong.
Let's take a look at what happened in the movie.
After Picard revealed that the Sona lied (in order to make the Feds not talk with these actually quite sophisticated people) and that the Baku are of the same race Dougherty wants to abort the mission (for the reason you spelled out, Dougherty cannot disavow the Prime Directive any longer). Ruafo them kills Dougherty and tries to to kill all the Baku in order to quickly get the particles. If it were not for Picard he would have succeeded.
DOUGHERTY: We're taking this ship out of here. This mission is over.
RU'AFO: It is not over.
DOUGHERTY: It is over!
RU'AFO: I do not take orders from you.
DOUGHERTY: If you launch the injector while the planet's still populated, the Federation will pursue you until...
RU'AFO: The Federation ...will never know what happened here.
So much about "zero intentions" and so much about the supposed ethical dilemma of the movie. As I say the entire time, there is none, the movie is utterly simple and the Sona are the bad guys.
I don't like simple villains either but I don't lie and claim that Nero is a good guy to make my point.
About the Baku being evil assholes who wanna keep everything for themselves, they never kill anybody, force anybody or even tell anybody to do anything. They accidentally landed on a planet which prolongs their lives and as they gave up technology up to a certain level they can hardly use subspace communication to tell the galaxy about the goldmine they are sitting on.
So you wanna seriously claim that giving up technology and not telling anybody about having found a fountain of youth is a horrible crime but actually killing people makes you the only good guys in this game? Seriously? Do all the Picard bashers somehow enter the Mirror Universe when they enter this thread?
As I already said, if the Feds had been aware of who the Baku and Sona were the proper way to act would have been to stay our of their conflict and contact the current leadership (which were the Baku when the movie started and assuming that the Sona reconquer the world could have later been the Sona) to make a deal about the resources.
I know that you come from a "relocate people to benefit the majority" angle and I would totally agree with you if this were an intrasociety issue. An example from Trek would be the relocation of settlers after the "land for peace" treaty with the Cardassians. I am totally for this. But in INS we deal with an intersociety issue which is a different ballgame. The UFP has authority over itself and thus its own citizens but forcing their will upon another people
in the name of the right of the(ir) majority is, unlike forcing your
settlers near the Cardassian border to give up their land for the more important common good peace, an act of violence and theft. The Klingons can justify raping a planet in the same way. We are trillions but you are just millions so give us your dilithium. Klingon children will go hungry if you do otherwise you selfish bastards!