Dastardly said:
To pose some challenging questions, I guess I'd have to ask why updating Huck Finn is a small deal to you, while something like the Thor change gets the green light. I don't have strong feelings either way, but glancing at these two highlights one of the problems with knowing the difference.
I'd say because changing Huck Finn would be altering the author's original intent (satire of prevailing attitudes), whereas the changes to Thor's secondary characters are not.
As an example, Thor's manner of speaking "ye olde english" was written to typecast him with a distinctive "style". That much isn't being altered in the movie (afaik). The skin colour of a minor god is being changed. It would be difficult to argue that this changes the behavior, mannerisms, or character of Haemdall.
Huck Finn's manner of speaking was written to parody period attitudes towards blacks. Changing the language uses completely changes the character's attitudes and world-view.
Put it in another context. Compare making the skin colour of a minor character a different colour, to changing him to be an arabian princess. One is a change of look that doesn't really impact the character at all. The other is a fundamental shift in behavior and mannerisms.