Table manners revisited

When I was taught table manners as a child, I learned that I should never cut potatoes with a knife. Manners make a lot of sense as they make life in a society more enjoyable for everybody. Though I never understood my social contribution by not cutting potatoes, until I was told that old cuttlery knifes corroded when they were exposed to the starch in a potato. This made sense, because of course nobody wants to put extra work on anybody cleaning oxidized blades. Strangely, I never questioned this rule until now. 

The photo below shows a high carbon steel knife of the manufacturer OPINEL. These knifes are very sharp and corrode by just looking at them. So, such a knife is the ideal object to test to impact of potato starch. The knife you see on the picture has been stuck in a potato after dinner for a whole night. Before that the knife was cleaned from all oils or other additives which could have protected it from rusting. But no sign of corrosion at all. Ergo, the origin of this rule stays a mystery for me.