I caught an interview with Gribben on NPR's "Talk of the Nation" last week where he justified his actions on two grounds:
Mark Twain was constantly returning to his work to edit it - his Autobiography is notorious for this. In fact, Twain said "[t]he difference between the right word and the almost right word is the difference between lightning and a lightning bug." So, according to Gribben, he is justified in determining the "right word" for modern audiences (perhaps channeling Twain's ghost?). If there were two or more readings of the text (as often happens with, e.g., Medieval manuscripts), an editor would be perfectly justified in choosing one interpretation over another (though they would be called upon to justify it). But here we have no disputed texts, no ambiguity about what word Twain thought appropriate (he uses it 219 times!), and Gribben is just plain, flat-out wrong, wrong, wrong to change it.
The second justification is that slave is a word with as many, and as powerfully negative connotations as the N-word. This last assertion is so wrong on so many levels it deserves no comment. While I don't believe anyone would enjoy being called a "slave," I think the reactions were you to go to an African-American community and call someone that would be bemusement and/or confusion rather than anger. On the other hand, were you to walk up to someone and say, "You lousy N-word!," the reactions would be quite different.
Marcia Alesan Dawkins wrote an essay at Truthdig, "10 Reasons Why the Slurs Should Stay in ‘Huck Finn’", which lays out the case quite eloquently. I reproduce here just the headings:
1. It’s Mark Twain.
2. Tampering with literatureis a censorship and it’s a bad idea.
3. Erasing racial epithets doesn’t erase race or racism.
4. It eliminates teachable moments.
5. Freedom of choice.
6. There were other options. Gribben could’ve rewritten the story from a different perspective.
7. You can’t fight censorship with censorship.
8. We’re not talking about the words that will replace nigger and Injun.
9. Offensive terms are being invented and popularized right now.
10. It’s coming from the “New South.”