Section 230 protects Amazon from this. They could still go after Amazon if they believe Amazon is negligent in fake user review controls. On paper I think Amazon probably does a lot, but probably not enough.
But this is not an opportunity of growth for Amazon. It is not a business critical area either and certainly is not perceived as meaningful threat or weakness.
More aggressive investments in this area aren’t justified because it truly doesn’t have a clear effect on the bottom line.
Reputationally, I doubt they care. Amazon has hundreds of thousands of vendors who sell up to 12 millions SKUs to hundreds of millions of customers.
It just makes sense to invest in tools and programs for the hundreds of millions of legit interactions (including reviews) that happen on their platform, than focusing too heavily on those who are not playing by the rules.
They are liable