Not a fan of the SEs or this change, but the usual reason given in favor of it is that Vader’s hiding information from Palpatine. He doesn’t want Palpatine to know that he already knows Luke is his son.
It raises a few more questions, though. Like, why would Palpatine care if Vader already heard about it? Or, why is the movie bringing this up when it’s never really relevant again? Plus, I don’t think that’s actually what Lucas had in mind when making the change.
But that explanation isn’t so horrible, and while I much prefer the original, it’s good enough for me. What bothers me more about the scene is Palpatine’s RotS makeup.
I wouldn’t call SE fans insane, though.
That explanation doesn’t really fly for me, since it takes a spectacular amount of obtuseness for Vader to not at least suspect that the Force-sensitive teenager who blew up the Death Star named Skywalker is his kid. Acting dumb just makes him look like a buffoon in front of the Emperor. Besides, since Vader is openly calling this kid ‘Skywalker’ from the beginning in an obsessive hunt, there’s no reason that the Emperor wouldn’t know his reasons for searching. Hell, I’m sure the paperwork on the probe droid operation would have been labeled ‘The Search for Skywalker - Probable Child of the Famous Jedi of the Same Name’.
Also, the idea that Vader would have some ill-will towards Palpatine for calling Padme’s death a few hours early seems like a stretch. It’s technically true that Anakin and his anger killed her, even if the details aren’t exactly as it seemed in ROTS. The fact is that Padme eventually died at Anakin’s hand and a child exists, something that isn’t impossible so Vader’s astonishment, feigned or not, is silly.