It's not a safety feature designed to deal with loony-tunes who might shove people on the tracks. It's intended to deal with accidental mishaps under very large, surging crowds.
Of course, in Japan you probably don't have to worry about surging crowds or anything like that. Just check out how orderly these people are while their fellow subway riders are crammed into the train in front of them:
These people are probably following a posted rule that says they aren't supposed to cross the white line on the platform until the next train arrives in the station. Imagine that ... following rules.