Quote: I think after alotta their previous games were ripped off by the bootleggers for the first game on the new platform they just protected the shit outta of it using as much of the protection devices as they could.
Not that it worked in the end as there are some bootlegs of Seibu Cup Soccer that made it out.
It's my opinion that the lack of encrypted program code in later versions of Raiden might well have been due to Seibu becoming aware that bootleggers were clearing that sort of hurdle; as mustangb and tdragonb demonstrate, bootleggers even appropriated the Raiden sound code for unrelated games after decrypting it themselves.
However, Seibu finally seems to have stymied the bootleggers by encrypting the sprites on most of their games from 1993 onward. There are sets of Raiden II/DX and the Raiden Fighters games which replace the original OPL2 or OPX synth sound with ADPCM-sampled loops, but they happen to be official rereleases even if they sound like bootlegs.