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.