SNAP is the acronym for the Supplemental Nutrition Assistance Program in the U.S., AKA "food stamps." It's overseen by the U.S. Department of Agriculture, which states:
SNAP offers nutrition assistance to millions of eligible, low-income individuals and families and provides economic benefits to communities. SNAP is the largest program in the domestic hunger safety net. The Food and Nutrition Service works with state agencies, nutrition educators, and neighborhood and faith-based organizations to ensure that those eligible for nutrition assistance can make informed decisions about applying for the program and can access benefits. FNS also works with state partners and the retail community to improve program administration and ensure program integrity.
To get SNAP benefits, you must apply in the state in which you currently live and you must meet certain requirements, including resource and income limits, which are described on this page.
SNAP enrollment peaked in late 2012. In December of that year, 47.78 million people received benefits ranging from $1,169 for a household of eight to $194 a month for one person. Since then, enrollment has dropped.
SNAP and earlier food stamps programs have been a target of cost-cutting measures by conservative politicians for as long as I can remember. Those who propose doing away with the program often cite "welfare queen" stereotypes and promote the idea that the program is rife with fraud.
Statistically speaking, SNAP fraud is rare. It's more common at convenience stores than it is as grocery stores, but it's rare, and it has been decreasing in recent years due to improvements in the system. There was a 3.5% fraud rate in 2012, and in 2017 it was a little under 1.5%.
Politicians are once again promoting plans aimed at reducing/eliminating/changing the program, and in response to the increased news coverage, there are people who work (or used to work) in grocery stores talking on social media about all the fraud they've seen.
Cashiers can be mistaken about what does and does not constitute fraud, and may take an unfairly judgmental view of legal transactions. Furthermore, fraud can be rare, but a cashier could still legitimately also remember seeing a lot of it.
Imagine a grocery store cashier checks out a customer every three minutes during an 8-hour shift. That's 160 people. Imagine that a quarter of those people are using SNAP benefits. That's 40 people. Going by the fraud statistics of 3.5% in 2012, that means that each day, it would be statistically likely that 1-2 of the SNAP users that cashier handled were doing something that cashier perceived as shady.
So, that cashier would literally see fraud (or what he or she thinks is fraud) every day ... but the fraudulent transactions would still represent a low percentage of the people who are using SNAP.
Furthermore, people's brains are designed to remember negative events more clearly than positive or neutral ones. If a transaction angers that cashier, he or she is going to remember that incident far more clearly than the other 100+ transactions they handled.