I don't think this is unethical. I don't know anyone who encourages unhappy customers to write reviews, on or off the app store.
It skews the ratings in your favor, deceiving future users about the actual quality of the app.
On the other hand, a private engagement directly with developers could prove to be more beneficial since it allows developers know of shortcomings/user-expectations without negatively affecting the overall ratings. This way, they can improve the app without having lingering negative comments affect the perception of their updates.
Well these aren't fake reviews that appear out of thin air. The reviews may be "biased" but they're still written by real, mostly happy, users.
For example, many people don't understand lots of apple's UI decisions that we take for granted. I had a user contact me and ask if our app did X. It does, I told him that, and he bought the app. Then I got an angry email saying it doesn't do that. (the thing was persisting data when "closing" the app with fast app switching.)
Well he thought all his apps were running constantly in the background draining battery so he force closed everything when he quit them. Obviously my app can't do the fast app switching persistant data if he force closes it. In no way does my app deserve an angry 1 star review when a simple email explanation can fix the problem.
I have one review in which a person said my app only allows 10 entries (it allows unlimited entries). One-star review, and I can't contact them to fix the problem. If I had a system like this implemented, likely they would have gotten in touch with me, I could have figured out what they were doing and solved their problem.
In that case, it would have eliminated bias by preventing an incorrect negative review. And helped resolve that person's issue.