It's in amazon's interest to suppress fake reviews, but it's definitely against amazon's interest to highlight the fact that their user reviews are often fake. The worst case for amazon is that users don't trust their reviews, and putting up a warning or score on reviews like fakespot doesn't improve the user's trust.

Amazon does police their reviews (as the article says, they've just sued another batch of companies for posting fake reviews) but they try to do it silently.