This is the same practice referenced in the name of the movie review site: http://www.rottentomatoes.com/
The suggestion is that your review might serve the same purpose as tomatoes you would have thrown.
The practice was common during the Vaudeville era, a time when over half the US population were farmers, and before movies and TV displaced live performances. Stand-up comedians still reference throwing rotten vegetables, their form being basically unchanged from their Vaudeville predecessors. So it is an interesting result that comic TV and movies seem to most reliably represent this practice. Everyone from the Marx Brothers to Robin Williams is represented in TV Tropes excellent list of examples: