What would lead Herb to fire one of his hired men?

Herb Clutter is very particular about the kind of people he hires to work on his farm. The first question he asks any prospective employee is if they drink. If they do, then they stand no chance of getting hired, as Herb has a zero tolerance for alcohol anywhere on his property. But even if they claim to be teetotal, Herb still insists on putting a clause into their contract that stipulates they'll be fired immediately if they're found to be harboring alcohol.
Herb's actions seem rather extreme and don't elicit much in the way of sympathy. Even his friends reprove him for his harshly puritanical attitude toward alcohol. One of them frankly tells Herb that if he caught one of his hired hands drinking, then out he'd go immediately—even if it meant his whole family would starve as a consequence.