If you limit reviews to only Amazon customers, you limited the number of people who can write reviews. I buy a lot of things from other places as well, not just Amazon.

It's to the point where Amazon really needs to re-investigate this policy though. They have enough sales volume that they can limit to verified purchasers and still have plenty of good content. Quality would trump the quantity too.

That's just one of a dozen things Amazon should be doing here though. We work with a few startups close to this situation, wow is it a mess.