It probably has something to do with the tampering of copy protection, which companies would very much like to be illegal. An arrest does not equate to a conviction; there's a chance the charges may not stick, but I don't know Japanese law.
While an arrest doesn't equate to prosecution, in Japan, prosecution almost equates conviction (somewhere around 99.9%).
DA's have a reputation to keep.
District Attorneys?
Well, closer to 99.7%[1]. But many less charges are filed, per capita, than the United States. This is partially due to things like honor and culture, but more to do with the fact that Japanese prosecutors have double the caseload of their American counterparts. And perhaps even more to do with the use of judges to decide verdicts.
It's also a false contrast. 97% of prosecutions in the US are resolved by plea bargain.