I know that very often restaurants and other businesses offer such a pathetic service that even a one star review looks too generous, but practically speaking, once you are used to the scale, it does not really really better what the high and low points are. Thinking of movies, I instantly know that a 4.5 is awesome on Netflix (5 point scale) and terrible on IMDB (10 point scale). Similarly, on Yelp, I know that a restaurant rated 1* is as low as it can get, even though the rating is not actually zero!
While from a viewpoint of a review-reader the choice of lowest rating point makes no difference, think of it from the perspective of a review-writer. How would you plan implementing a zero star rating? Currently, Yelp asks you to both write a review and pick a star-rating. So, an obvious solution for submitting a zero-star reviews is to post the review without selecting any stars.
But think of the inadvertent mistakes where after writing a raving review, a user forgets to select the star rating! (and believe me such mistakes will happen!) So, the restaurant gets the lowest rating, while the user in fact wanted to give them the highest! This may prove to be disastrous, as many Yelpers (me included) start with the average rating of a business. Only if that is reasonable (3.5 or higher), then, I proceed to read the reviews. I know an easy solution to this problem is to ask the user with a popup if he really wants to post a 0 star review, but as discussed in first part of this answer, the additional implementation effort may not be worth it. Instead, Yelp safeguards against this problem by forcing you to pick a non-zero rating before you are able to submit a review!
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