I see two possible reasons for such a statement. First, it could be that traditional home cooks are accustomed to salt vegetables on a very rough basis, using some homemade rule like "a teaspoon for one pot of soup", and potatoes being blander (less sour, less aroma) than many other vegetables, they prefer to add more salt to potato-heavy meals to achieve a similar level of seasoning-ness.
The second could be a logical derivative of the idea that adding potatoes fixes oversalted soups. If they are an antidote for salt, they need more of it to reach normal saltiness when added deliberately, right? Only the problem here is that they are not a magical anti-salt thing at all. They don't really "fix" the soup, they are just a convenient way (cheap, bulky) to provide dilution in the salty soup, reducing the average saltiness of the chewed potato-stock mixture in the mouth.
Both are not really good reasons to stick to the rule. The second one is plain wrong, and the first is overly simplistic. The better way is to adjust your seasoning based on all factors of the recipe, not just the presence of potatoes. And always taste test when adding seasonings.
So, to sum it up, there is no such reason at all. Your assumption is based on misunderstandings and kitchen myths.