Well, while the commission is 0, there is the small interest payment of between 2 and 3% depending on the broker. So that would be just 30 to 40 yen per trade for the average small lot trader. But the brokers are also interested in gaining market share by offering 0 commission, and hope that the customer will also trade other products, which seems to be working as they've been offering this for years.
you are talking about the margin rates? Why do I have to incur 2-3% for day trading?