He's only been here a week, and the guy who he replaced had only been here two months. The guy before him had been here four full months, but a robbery scared him out of convenience store work and back into school. A gun pointed at a guy's head makes him think about career changes pretty seriously. A high turnover of help like this means the night guy doesn't know his job well enough to do it correctly. Even if he does know that he's supposed to toss the bad milk and the bad beer, he usually doesn't, because he's so worried about guys pointing guns at his head.
Milk expires after two weeks on the shelf and beer expires after three months.
Not many people know beer expires, but it flattens and sours like anything else. There's hardly ever beer to toss because we move it so quickly. But the Pabst Blue Ribbon usually goes bad. No one drinks Pabst except the owner, so he buys ten racks of Pabst a month. For some reason, men never buy milk and women never buy beer.