Well, it's not considered canon because it was widely circulated, but because it was stated to be canon by everyone involved. (But has apparently now been retracted, causing headaches for most)

And my understanding is that you end up with more plot holes, not less, if you start reading the Star Wars books or watching the cartoon.

But the curse thing isn't something that that you're expected to read/watch other things to see and understand, but an official(if now retracted) explanation of something that was poorly explained in the movie itself(which doesn't excuse the poor explanation in the movie-the curse was never remotely explained well-but that is why it's considered canon)