I think this is classic passive-aggression, he's obviously pissed, but rather than insult anyone outright he basically says "if you buy Before Watchmen, you are not worthy enough to purchase anything I write" which is ludicrous, how does he even know that the prequels will be bad? he doesn't, he just hates them on principle. he also pints out that it's (OMG) 35 books! but that's only a drop in the ocean compared to other book series.
Be honest, if he had insulted anyone you'd still be having the exact same opinion of him, no merit whatsoever.
And how is it wrong to hate something in principle ? Would you wait to see (if it's ever announced) how Watchmen 2 would do ? Even if it actually was one of the worst ideas to ever be conceived in comic book history ? Some ideas are just BAD, regardless of how good the prequels are, they are still unnecessary literary masturbation.
And while I severely disagree with Moore's conception of who is and isn't worthy of reading his books, he is partially right, if all someone got out of the Watchmen books is "OH MY GAWD RORSCHACH IS AWESUM ! GIMME MOAR RORSCHACH !!!", then quite frankly you're not mature enough to read "V For Vendetta" and "From Hell".
He should just be happy that he has created characters and a universe so beloved that almost 30 years later people still want to see these characters, that only existed in a 12 issue series, in more stuff.
You wouldn't be so happy if you suddenly discovered the people to whom you trusted your work with suddenly gave you a middle finger as soon as you turned your back on them, and even if he was partially responsible for that outcome, being naive doesn't take the spotlight from being an unethical businessman.