If someone views a member profile sure they can add feedback as long as they are a registered user.
The people who are sent the direct link are usually the buyers.
You can not change how this behaves without modifying the member.php files and profile template
Easiest change would be for you to try and remove this from your profile template but this is unsupported and use at your own risk. Code modification is not part of product support.
Code:
Content visible to verified customers only.