All of these fields are artificially irrigated. One need only contaminate the irrigation source. A much more likely source for Salmonella than human is contamination with animal waste. Swine and poultry are the most likely sources.
I'm not saying that it's impossible to get sick from an illegal pooping in a field somewhere, just that the scale of the outbreak essentially eliminates it as a possibility here.
Extra info: excerpts
Food perils greater in global society
Foodborne illness statistics are staggering. The Centers for Disease Control and Prevention estimate that each year more than 5,000 Americans die from foodborne illness, more than 300,000 are hospitalized and another 76 million get sick.Globalization of the food supply, which enables shoppers to buy fresh fruits and berries in the middle of winter, also presents the potential of allowing pathogens from developing countries entry into our food supply. For example, Cyclosporia cayetanensis, a unicellular intestinal parasite not reported as a cause of disease in the United States until 1995, has recently been responsible for a number of significant foodborne-illness outbreaks related to raspberries imported from Guatemala. There was also a nationwide outbreak of hepatitis A related to frozen strawberries from Mexico, distributed from a San Diego company to 16 state lunch programs.
The Food and Drug Administration inspects approximately two of every 100 shipments into the United States. Perpetual staffing shortages and an ever-increasing workload prevent the necessary inspections that these higher-risk imported foods require.