Eating cheaply, and spending the per diem on something else isn’t just “technically” not a fraud. It is a perfectly reasonable, and completely legal thing to do.
Most organizations (private and public sector) pay travelling employees per diems because it’s actually cheaper than requiring itemized accounting of all expenditures. If you are given (say) up to $50.00 for a meal allowance, and have to submit a receipt — then you’ll tend to spend $50.00 per meal. If you receive a per diem of $40 per meal, and spend only $30 — then both you, and your employer come out ahead.
Up to the applicable Federal per diem rate. There are quite a few rules, including being paid under and “accountable plan”, etc.
http://www.irs.gov/publications/p463/ch06.html#en_US_2011_publink100034177
Some employers and employees may be way off the mark on compliance but not realize it or really care unless they’re audited.