alt Hacker NewsTwo problems:
1) Michael Douglas is extremely attractive and charismatic, so people like him even if they're not supposed to. Gekko seduces the viewer to the dark side like he seduced Bud Fox.
2) The movie never shows Gekko in jail, so you never see the true consequences.
Prison sentences were so short back then, plus parole, that it was arguably worth it. it worked for Boeseky, Milikin and others.