[_delirium]

In some cases it may be illegal in the U.S. also. The clearest case would be if a direct competitor hires someone to post fake negative reviews intending to damage a competing business (and fails to hide the trail). In that case the "target" would have a good case that their competitor is engaged in some kind of tortious interference with their business.

edit: There's at least one case currently in the courts on a similar theory, http://tushnet.blogspot.com/2015/02/allegations-of-fake-inde...