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.