It is very common for sites to allow non-verified purchasers to leave reviews. Amazon works this way. You'll see reviews on Amazon all the time where people specifically mention that they bought it somewhere else. I've always wondered why someone would leave a legitimate review on Amazon for a product they didn't buy from Amazon. Maybe the answer is that they aren't legitimate though.

Some review software notes when a reviewer is a verified purchaser. I look for that when reading reviews because at least they have the product in question.