I have a feeling that most people equate copyright infringement with something akin to cheating on your taxes, you get away with it 90% of the time as long as it isn't something massive. Tax authorities will dole out some wrist slaps and get a lot of publicity for it so the action will inhibit those in doubt. Tax authorities are acutely aware that putting tax evaders in prison is essentially a loss-loss situation, no tax income from that individual but expenditures for the state.
I believe the Japanese government instituted these new draconian laws with the explicit intent to create some high visibility "wrist slapping", they will go after young punks, already living on the edge of society, already committing hard to prove crimes but being sloppy downloaders of free digital content. They caught Al Capone on tax fraud, this is most likely something similar.