First, the sick old men lie. Employees are strictly forbidden to buy them alcohol, so the grandfather-neighbor ordered the alcoholic tincture from the pharmacy to supposedly drop in the ear, and he himself diluted and drank.
Secondly, the fact that the girl has a grandmother who usually buys vodka for grinding does not guarantee that at this particular moment the girl is telling the truth and did not come for the bottle for the thirteen-year-old spinach from the yard. It is convenient for her grandmother to know there - she will be sold to circumvent the rules; neither the inspectors nor the seller could know this.
Third, what prevented the grandmother from pursuing a duty therapist and sending the child to the pharmacy for an alcohol rubber?

In Pratchett’s last translated book, two trains collided on a single-wheel, because a kind of benevolent clerk “just wanted to save everyone’s time” and gave the symbol of “preferential passage” to all trains at once.