The problem is that people that support these kinds of measures are under the delusion that certain fantasies lead to real world results.
They do this whilst blindly denying that other kinds of fantasies do not lead to results.
e.g. supporters of this move believe rape games make people rape (if they don't believe this then what is the problem?)
However at the same time a large number do not believe that violent games where you kill people lead people to want to kill people (whether that's by murdering people on the street or joining the army to murder them for some political ideal)
end of examples.
This "interesting" belief is no different to the extreme porn law foundings, that have less than zero evidence (any evidence in support is easily disclaimed - the only support coming from common sense narrative and prisoner survey) whilst the evidence against is convincing.) Or the notion that strip clubs increased crime (while statistics showed crime decreased in areas of London.) This can be extended to the lolicon debate with zero effort.
This is quite overtly a case of moral outrage against something people view as abhorent, whilst serving no purpose other then to punish the few to inflate a handful of egos.
As to crime rates in Japan all measures (quantative and qualitiative) point to secual offence being far less of a problem in Japan that it is in other modern nations - including my personal experiance of talking to Japanese woman.
Interestingly over the course of today I've decided Japan is an increadibly interesting nation to direct further studies, so once I finish my degree I feel it's a place that has many an interetsting social phenomena to study.