If you have a per diem, you don't turn in any receipts. You can expense a $50 per diem and spend nothing in reality, and pocket all $50 if you want. Tax-free, too.

However, if expense $7 with a receipt that you didn't pay $7 for, you are stealing $7 from your employer.

No, $7 is not big, but you yourself stated that if you do this on every flight, it will add up. Not a good idea if you ask me.