Yelp = Yellow Pages. It's an online version of the Yellow Pages that allows people to post comments about the company who is advertising. They don't have any control over who posts, and based on the recent posts there seems to be quite a flame war going on between those who support the owners and those who do not. Link
They may not be able to control who posts, but they can sure control whose posts remain where.
The side against the store is quite vile.