I think Amazon's rating/review adjustment is entirely warranted in such a case. Why should a (presumably) honest seller get dinged because someone else added bad stuff to Amazon's inventory.

The problem is Amazon's pooled inventory that leads to this. Even if they continue to pool, each individual item should be tagged in Amazon's records with the seller that provided it, so that when there are issues, Amazon can go back to the provider (and ban them after repeated or blatant problems).