Collegeguys4 and randybishop are both correct. Amazon allows you to review items you've purchased but haven't bought on Amazon because the more reviews a product has the more people will buy it through Amazon.
Most people are not going to look up a product on Amazon for reviews and then go buy it at their local store. Most of the time if they're reading Amazon reviews, it's because they plan on buying that product on Amazon. Therefore, those reviews that aren't verified purchases are just icing on the cake.