I agree, it is totally unethical. Although it didn't happen to me, it happened to one of my colleagues who was asked to use at least two references from the same journal.
How can we allow the other party to suggest in academic matter. Our relationship with publisher is only giver and taker. Also copy right with author only. We must be in a policy to determine the things on our side. The question of ethical or unethical will not be arises, if we confident on our work, the interference of publisher is insignificant.