I have lived in Japan for only two years, but may have more experience than the author of the letter ("Pocket knife lends tourist, 74, in lockup") appears to have of the culture and life in this beautiful country.
Regardless of your country of origin, if you wish to travel to another country, don't carry a dangerous weapon — namely a concealed knife. It is ridiculous to think that the Japanese police, or any police for that matter, are at fault when they arrest somebody for carrying a concealed weapon, regardless of the age, gender or nationality of the offender. Criminals come in all shapes and sizes, and it's nobody's responsibility to expressly explain all the laws of a country when one enters it. Whatever happened to personal responsibility?
It may be common practice for people in the United States to carry concealed weapons to protect themselves from each other, but it is not common in most other countries, especially Japan. The offender is described as appearing old and gentle — who wouldn't hurt a fly — but this view is from somebody who knows the offender and, in all likelihood, is of the same country of origin.
Nine days in lockup is a long time, though. The offender won't be doing that in a hurry again.