THE decision Monday by Iran-contra special prosecutor Lawrence Walsh to drop the charges against Oliver North was a victory for the former White House aide. Was it a victory for the American system of justice? No, and yes.In one sense it's a miscarriage of justice that a central figure in the Iran-contra scandal cannot be successfully prosecuted. Mr. North lied to Congress, shredded documents, and took other actions to conceal his secret network in support of the Nicaraguan resistance, which operated in violation of congressional mandates. North's assertion this week that he has been "exonerated" is mere bravado, given the conviction on three criminal counts handed down by a jury of his peers in 1989. But North's conviction has been set aside on appeal because of the prosecution's inability to prove that all the evidence against him at trial was totally unaffected by North's testimony to a congressional committee in 1987. The lawmakers probing the scandal granted North immunity in exchange for his testimony, which otherwise would have been shielded by the Fifth Amendment. Ironically, North is free thanks to one of those infamous "technicalities" that law-and-order conservatives rail against. Yet those "technicalities" - like the right against self-incrimination - are the glory of a justice system that has steadfastly resisted pressure to slide into star-chamber practices. Perhaps such people will now appreciate the technicalities of a system that deems it better that the guilty walk than that the system itself be unfair. The dismissal of the charges against North demonstrates the nearly insuperable challenge prosecutors face in convicting defendants who have previously testified to Congress under grants of immunity. Sometimes, in the interest of swift disclosure of wrongdoing, Congress may be justified in jeopardizing subsequent law enforcement. But that's not a decision to be taken lightly - a fact Congress needs to remember as it investigates BCCI and the Salomon Brothers financial scandals.