Ethical? I would say yes. Notice that they are not fixing the polarity or the content of the reviews. They just change the language to be correct and error-free. I can see the counter-argument that the writing style allows us to judge if the review is serious or not. So, artificially improving the writing style may be considered as interference with the perceived objectivity of the user-generated reviews.

But is it ingenious? Yes! It is one of these solutions that is sitting in front of you but you just cannot see it. And this is what makes it ingenious.