I agree that I think it's lame that these actors won't further embrace the roles that made them famous. Especially when all they would have needed was some voice over dialog, that could have probably been recorded within a short afternoon. I'm not expecting them to do comic cons, and attend the grand opening, but at least make sure the character is represented well.
As someone else said, they will be recognized as these characters there entire lives. There is no going back from that. It's not like Universal will stop using the Actors likeness anytime soon. They are there forever, so why not actually care enough about your character to make sure the vocals sound correct?
it's just incredibly pompous, but hey they are in their 20s and rich, so why should they care? Maybe one day they will be desperate for a paycheck and will re-record the audio for universal on the 10th anniversery of the ride or something.