The inhabitants of the planet were Maquis, not Federation. They also evacuated in time before the trilithium resin affected them.
Both of these claims are dubious.
"For the Uniform" is the first time anybody refers to the concept of "Maquis planets". Until then, the Maquis had been a paramilitary organization operating on several locations, including planets in the Demilitarized Zone, planets in regular UFP space, planets in neutral space, locations in Bajoran space, and apparently (VOY, esp. "Non Sequitur") even Earth. By that definition, Sisko would have been justified in bombarding Earth!
But let's assume for assumption's sake that every child and cripple and pet rodent on that planet was a fiery-hearted Maquis terrorist. Sisko meets Eddington in space and says he is going to bombard the planet with a few kilograms of a highly poisonous chemical. Eddington does not believe him, and the odds that he would have relayed Sisko's threat down to the planet are slim; Sisko himself certainly did not contact the planet, and only traded threats with Eddington. Sisko then fires the poison torps anyway. And sure enough, within minutes or less, people down below begin evacuating. Why? Surely not because they had advance warning (because they had none). Surely not because some atmospheric researcher somewhere on the planet (do fiery-hearted Maquis terrorists have those?) saw a needle twitch on her instrument for detecting extremely rare poisons (because who listens to people in lab coats, much less understands what they are talking about?). Surely not because dense clouds of evil-looking substance blocked the sun (because the torpedoes only carried a handful of the substance, not enough to create visible clouds). Virtually the only possible explanation for these hardy settlers and merciless combatants to uproot themselves in a matter of minutes is if people around them began to die in horrible convulsions. Not cough and turn slightly greenish, but die, and die horribly. Otherwise, these people would just stiffen their upper lips with a hypospray, batten down the hatches, and wait for their ringleaders to inform them how to best fight back.
When a planetwide evacuation is inspired by people dropping dead, it's guaranteed to fail. If these Maquis terrorist grannies and murderer babies all carried TAS style life support belts or Israeli style gas masks every day, every hour of their normal lives, then it's possible that relatively many of them would make it to the evacuation ships (which a Maquis terrorist fugitive planet might indeed have for 100% of the population, even if no other semi-stable colony would conceivably have the reason to prepare that way). If no portable personal life support was available, the evacuation would become less and less efficient by the minute, and mortality would surely approach 90% at the very least.
As explained in almost every episode since the Maquis were developed, they used to be Federation citizens, but they all gave up their status as Federation citizens when they refused to leave the DMZ.
Umm, nope.
In TNG "Journey's End", a single planet in would-be Cardassian space (not the DMZ, which wasn't even on any drawing boards yet, but Cardassian space) decided to abandon UFP membership in favor of Cardassian Union membership; every citizen there became a Cardassian citizen. In the later and more or less simultaneous TNG "Preemptive Strike" and DS9 "The Maquis", a DMZ had been formed, and consisted of UFP planets and Cardassian planets, all of them respecting their native law but in addition agreeing not to host any sort of a military force. There was no mention of neutral planets or third-party planets there whatsoever.
In "For the Cause", when Eddington defected for good, he said the Maquis had left the UFP.
"Starships chase us through the Badlands and our supporters are harassed and ridiculed. Why? Because we've left the Federation, and that's the one thing you can't accept."
However, in "Blaze of Glory", Eddington (observing the carnage of the last remaining Maquis) lamented that the Maquis were only considering declaring themselves (that is, entire planets of theirs) independent from the UFP when the Dominion came and wiped them out.
"We were winning. The Cardassian Empire was falling into chaos. The Maquis colonies were going to declare themselves an independent nation."
Apparently, then, Eddington in "For the Cause" is only saying that the Maquis criminals have left the Federation, not that their "supporters" (the expression he uses, apparently referring to the colonists whose hospitality they exploited) would have left.
That means that everybody on the Maquis planets, including the criminals and their noncriminal supporters, was subject to UFP law and respectively deserving of UFP protection. Staying in the DMZ would not remove UFP protection nor the obligation to observe UFP law to the hilt. Becoming a Maquis terrorist would. (That is, a criminal would of course not obey the law - but he would still have rights within the UFP legal system).
Timo Saloniemi