It's because waiters are writing books saying they are spitting in people's food that people believe they are having their food spit on. If you want to blame somebody for spreading that information (true or not), blame the wait staff.

From the NY Post

STEVE Dublanica has swapped your decaf coffee for regular, "crop dusted" your table with his intestinal gas and called the cops on you after you got drunk and staggered out to your car.

Lesson No. 1: "Waiters can and do spit in people's food . . . I prefer more elegant methods of revenge."

Dublanica is a waiter. Actually, he's "The Waiter" behind the anonymous (until now) four-year-old blog Waiter Rant, and he's got a new book of the same name out in stores today chronicling his nine-year career waiting tables in the city's affluent suburbs (he won't divulge the true identity of "The Bistro"). Anthony Bourdain has called it "the front-of-the-house version of 'Kitchen Confidential,' " his stomach-churning restaurant exposé that changed the way New Yorkers ordered food.


Read more: http://www.nypost.com/p/lifestyle/fo...#ixzz1urePvwo8

Of course he's also said he's never seen it done, but most people only remember him and others warning you they do it. Headline stories like this one don't help the issue either

From Fox South Carolina McDonald's employee arrested for allegedly spitting in tea