i can't imagine many people would agree with how you're using the term "rational". short-term thinking is not an automatic outcome of rationality... you seem to be describing some of the negative effects of capitalism instead.
"Short-term" is also relative. Reading what the original author wrote, I interpreted it as them working backwards from the severe punishments for underweighting bread and what might cause them. Perhaps it's not the original baker underweighting bread, but their son or their grandson who doesn't have the right context for not doing so. Maybe they are trying to get more out of the family bakery, try something new, shake things up. It might be rational in the sense of game theory and the expected outcome of multiple generations of bakers. So over 100 years time you might find half your bakeries have reduced the size of their bread while the competing town has not and they overtake you.
Or maybe the town faces hard times and the baker might want to cut costs by reducing the weight of their bread to help their own family. That might be rational under those circumstances.
