Sure, but it would surely reduce the scale of these fake reviews. Right now, companies can buy reviews in batches of thousands; the only actual cost is in somebody/something creating that review and some overhead.
If Amazon also required actual purchase of that item, now there's the extra cost of actually buying that item. Even more so when you're talking about things that are expensive. This would drastically lower the amount of spam that could be generated unless the company wants to throw some huge money at it.
Returned items would still potentially be an issue, but I'm guessing Amazon already has things in place to kick off users that return too many items.
If Amazon also required actual purchase of that item, now there's the extra cost of actually buying that item. Even more so when you're talking about things that are expensive. This would drastically lower the amount of spam that could be generated unless the company wants to throw some huge money at it.
Returned items would still potentially be an issue, but I'm guessing Amazon already has things in place to kick off users that return too many items.