> Why don't they only let you review items that you have purchased using the same account? Too simple I guess.

Doing that would help, but probably not as much as you'd think. A lot of the fake review scams have the reviewers purchase the products legitimately and then get reimbursed through Paypal.

They do highlight reviews from users who actually purchased the item. That's why there's a thriving business in free products in exchange for good reviews.

Giving away a few hundred or thousand items can lead to huge sales numbers.

The idea is that people may have bought it somewhere other than Amazon and to let them post a review. Amazon does distinguish between “verified purchase”s and not