Even if those places were under the control of the Empire, they wouldn't be celebrating anyway. First of all, the Return of the Jedi Special Edition ending implies that the celebrations are all happening the same day. There's no way that even news that big would be able to get to all those places as fast as it did. Second of all, even though the Empire's ultimate weapon and two biggest bad guys were destroyed, it doesn't mean that all of the Imperial troops all across the galaxy magically dropped dead like the droids did in Episode I. Just because the Empire's main sources of power are gone doesn't mean the troops died. They'd still be around and able to control those places and I honestly don't see why everyone would be celebrating, unless they managed to kill all of those remaining troops after hearing the news. And besides, Death Star II isn't the only place where the Empire is stationed. There are obviously other bases around the galaxy and even if Death Star II was the only one, the Empire would still be spread out around the galaxy and it would take a lot of time to get rid of all the remaining Imperial troops.

And even if the celebrations were just for the destruction of Death Star II and not the end of the Empire, there'd be no reason not to have similar celebrations for the destruction of Death Star I. After all, the first Death Star had been completed and had actually blown up a planet and would've destroyed the Rebel Base if it hadn't exploded a few seconds earlier. It was a much bigger threat than Death Star II, which didn't even get completed and only destroyed a few rebel cruisers.

Sorry, but the Return of the Jedi Special Edition celebration ending just doesn't make sense and is really pointless, since the various planets wouldn't have total freedom yet.