James Earl Jones wasn't credited for 20 years for his role in the first two Star Wars films. It seems strange, since it's become such common knowledge who the voice of Darth Vader is (and likely was back in the 70's as well). This was at the request of the actor himself, since he felt his contribution wasn't significant enough to deserve credit. Still - even watching the credits - you wouldn't assume that iconic deep voice was that of Bristol bodybuilder David Prowse (who was physically Vader). It's not like everyone was scratching their heads for 20 years, asking "Who's voice was that?" Regardless, after 20 years and at no one's request, they figured they'd put the name back in.
Still -- the reason they left it out initially was due to James Earl Jones' specific request. The only reason to put it back in would be if Jones asked them to put it back in, right? Given he still remained uncredited in Revenge of the Sith, that sounds pretty suspect. I can't say for sure, but it's hard to picture James Earl Jones sitting in his home, stewing at the thought that his work wasn't being properly recognized, and calling up George Lucas and demanding his name be put back in. I do kind of understand not wanting to be associated with Revenge of the Sith though.
Yay! Bust out the space bubbly and let's get this galactic party goin', 'cuz the horrible, oppressive Empire is gone! Well, except, uh, it isn't. Sure, a big portion of their strength is gone - and most of the leadership! But if you think every single Imperial officer and soldier was at that battle, you got another thing coming. There are still going to be a lot of Stormtroopers on all of those planets to "keep the peace." A planet like Coruscant would be far more likely to descend into chaos and anarchy than a well-organized planetwide party with fireworks going off. And if you don't think some other Imperial officer is going to take over where the Emperor left off, you're crazy. Plus, wait, not even every ship there was destroyed! There were still some major Star Destroyers in good shape!
And beyond the fact that the Empire is still around and there would probably be rioting and violent uprisings everywhere, maybe not everyone would even think about celebrating - the Empire seemed to be the largest employer in the galaxy by a sizable margin. Can you imagine how many Stormtroopers were required to man the Death Star and a dozen city-sized Star Destroyers? Odds are you had a friend or relative on the Empire's side in that battle, and now they're dead. This is not a time for celebration, everyone. There are like 50 million space funerals to plan.
Okay, even though George Lucas wasn't the credited screenwriter or director of Empire Strikes Back, I like to assume he at least understood the film. Like, on the most basic level of character motivations. He knew why Vader wanted to get Luke so bad (he was his dad, turn the budding Jedi into his Sith apprentice), he knew why Leia wouldn't admit she loved Han (because he was a scruffy nerfherder), and he knew why Luke plummeted when Darth Vader told him to join the Dark Side so they could rule the universe together (he'd rather be dead than join Vader). Adding a terrified scream to that fall seems to indicate he thought Luke slipped or something, instead of stoically sacrificing himself. Why would someone bellow in terror if they were doing this on purpose? Maybe you could argue that, while Luke knew what he was doing, the reality is actually pretty scary. Well, you're probably wrong, since Lucas doubled back and removed the scream.
Here's the really weird part though: the scream wasn't even Mark Hamill's. It was the Emperor's scream from when HE plummeted to his demise in Return of the Jedi. Weird, right? Like they didn't have any recordings of Mark Hamill screaming? If they really didn't, why not have him record one? Or, if they really wanted get a genuine scream out of him, just tell him what his future career as a movie star looked like.