The traditional Japanese Straight was being used long before western style straights were introduced into Japan. Very few Japanese in the old days went to barbers for shaves the same as most westerners. Most shaved themselves.

Its like I have always said when we come across something that seems alien to us we try and think of some complicated reason to explain it instead of just accepting the obvious. In this case the obvious is they learned to use the straight the same way we learn to use a Western Straight.