Not only do they delete reviews (I'm sure you can find thousands of complaints online if you google it), they also allow sellers to take products with 5 star reviews and repurpose them for a completely different product, reviews and all, and many 3rd party sellers will bribe you in exchange for a positive review (e.g. free products).

Good point. Although on several occasions, I've left a negative review on a top rated product, only to have the seller contact me immediately in an attempt to bribe me to delete it.