If a photo is actually a combination of several photos, but the photographer implies that it is a single, unaltered image, do you consider this unethical?
For me, this is the main issue. Mind you, he never explicitly says that the image is one exposure (in fact, I believe someone dug up a press release where he outright says it's two*), but the corny story behind it strongly implies that it was somehow a "magical instant" when that instant couldn't have existed. Mind you, the moon could have by itself (as could the silhouetted tree), and that alone is actually, in my opinion, the most striking and memorable part of the image.
Now, don't get me wrong. Peter Lik does some nice work, and as a fine art photographer, in my opinion he has every right to make images for the sake of art rather than documentation. It's the attempt to make the image seem like something it isn't that is bothersome.
By the way, I've been to his galleries in Las Vegas, and I have to say, more than the images themselves, the presentation is jaw-dropping. I know quite a few people with enough talent to take pictures as good or better than the ones he takes, but I know absolutely nobody who can beat his marketing. And at the end of the day, all that matters in business is the ability to get a sale.
So in short, as a fine art photographer, I believe compositing images is 100% ethical. Misleading the potential buyer into believing it was a single exposure is not.
*P.S. He uses the phrase "double exposure", which refers to something completely different. This image is a flat out composite.