Even if they didn't, how would we know? I have seen countless 1-star reviews of restaurants that I have visited where the content of the review is just bizarre: incorrect names of people, items that aren't on the menu, server practices that are completely out of the ordinary, etc. I can't help but think they are the result of a desperate Yelp ad salesman who wants to burn someone for not buying.

Might be nice way to get a competitor ranked lower - whereby hiring people to create bad reviews of your top competitors ...

Yes. But only if you believe the near endless stories of available online.