By that logic, why do we need to legislate against murder, rape, theft, assault etc? They're all morally wrong, so why should people do it? Do we really need government dictating that I can't kill a guy because he makes idiotic counterarguments on Slashdot?
So to sum up, why do people take advantage if they can? Because there is an advantage to be gained.
And as for that sarcastic diatribe about 'innocent' bankers... dude, I didn't say they were innocent. In fact, I explicitly stated that they were not blameless. The issue is not binary, there is no absolute wrong and absolute right. You need to get your head out of your ass and realise that reality is not that simple.