You make a good point. While the reviews are real reviews, written by people have bought/experienced the product, it can be distorted.
I don't believe there is anything wrong in asking customers to leave you a review, by e-mail, wall poster, or letter with the product. But it becomes a problem when they are told they will be given an incentive to leave a review, via a refund or free night next time they stay.
However, there is nothing wrong in giving them a free night, or refund if they are not told about it in advance.
This prevents the reviews from being distorted, giving it a more balanced view.
But put it this way, if you are a busy hostel (let us say 250+ beds), you will be getting constant reviews, and no matter how many fake/inaccurate reviews there are, the stats will show you are a good hostel, due to so many people reviewing.