Actually, I'm of the opinion that it's about Federation citizenship. So long as the inhabitants of Dorvan V were Federation citizens, they were going to be forcibly removed, because it's Starfleet's duty to protect Federation citizens, & adhere to treaty stipulations
But the inhabitants of Tau Cygna V aren't Federation citizens... technically. Originally they were going to be a Federation colony somewhere else. They weren't even known to the Federation, & that expedition was considered lost.
I suppose once they knew of them they could have claimed it as Federation. The Sheliak certainly considered it such, but if they didn't capitulate, they really have no jurisdiction to remove them forcibly.
In fact, at the end of Journey's End
, The Dorvan V inhabitants seceded from the Federation. Ultimately, the Cardassians could have just executed everyone on that planet, 5 minutes after the Enterprise left orbit, despite the Cardassian commander's assurances, & they'd have had no jurisdiction to do anything about it.
So the answer is that Starfleet couldn't forcibly remove either colony, if they renounce their Federation citizenship, but if they are considered citizens, then they are able to be forced out by the Feds