Sure, but I don't see how this is significantly different than if I had gotten dinner at a restaurant. If I had bought food at a restaurant, and even if I got a dessert, that wouldn't have been questioned at all. For that matter, if I had gotten fixings at a grocery store to make sandwiches or something, that also wouldn't have been questioned. Every morning when I would drive to work on these trips I would buy a soda at a gas station, and that was never questioned, even after adding the receipt.
Fiber One bars have extra fiber, but ultimately they really are just glorified candy bars, not radically different than a dessert. I don't see how this was significantly than ordering a dessert at a restaurant.
Because the law is complicated and they were erring on the side of caution to stay within a relatively well-defined safe area to avoid committing tax fraud.
They did not spend hundreds of dollars in labor to save 5$. They spent hundreds of dollars in labor to avoid committing tax fraud while soothing your self-admitted petty ego.
If the government audited the company's tax records and disagreed that the fiber bars should've been expensed, they'd most likely charge tax, interest, and a penalty proportional to the $5 cost. Fraud requires intentional wrongdoing; no court or auditor is going to find that a company intentionally schemed to defraud the government of a couple dollars by sneaking fiber bars into a travel expense report.