Why do people assume they can get away with things like this?

Because, more often than not, they do. My former boss was the king of deception. He'd go to extremes to avoid playing a bill or cheat someone. He'd have someone Photoshop steel tags so that he could go back and debit the supplier for the weight difference, he set up three different companies on paper and had a phone bank, so that if someone wanted credit references on one of his companies, they other two would give sterling ones. Just a few of his many tricks.

I've known of this before. But the company caught the employee- because they pay attention to a trivial thing known as the budget....