Well, as a guess, I don't think the Empire wanted Belsavis. Because the Empire only wanted a few prisoners out of there, make every situation as miserable as possible for the Republic and the Emperor just wanted death and destruction.
Now after the known arcs, they got there prisoners, they've retreated when they "lost" the war and the Emperors plans have failed.
The Empire has, apart from Bonus and Section X, nothing more of interest on Belsavis. But because Section X doesn't really have a story, like many dailies, part from Oricon, I don't think you should put much thought into the fact that they are still present on the planet other than for dailies.
Therefore it is safe to assume it got back under control by the Republic.