By and large, the government doesn't put poor black kids in jail without proof. A lot of my friends are public defenders working with these sorts of people, and by and large they commit the crimes they're accused of committing. The problem isn't that the government jails these people without proof, it's that you and I as voters, along with "just say no!" moms, etc, have continually voted for "tough on crime" laws that punish these people vastly out of proportion with what they did. But they did do it, and the police have proof because it's easy to prove "poor black kid" crimes. It's easy to prove it with evidence like "you tried to sell drugs to an undercover cop" or "we found the stolen car at your house."
With white collar crime the situation is different. Those laws make otherwise innocuous acts and omissions criminal to achieve broader social purposes. There is nothing intrinsically wrong with saying an incorrect fact to a shareholder, or making a killing on a lucky trade, or getting the better end of a big deal. All those things only become wrong with proof of criminal intent (you knew the fact was wrong and intended to defraud investors--you didn't just make a mistake or restate a bad projection, you had inside information about the trade--you didn't just get lucky, you didn't disclose information you knew to be material to your counter-party).
We require intent for these things, because they are otherwise quite ordinary. They happen all time. People make mistakes all the time. People get lucky on trades all the time. People get the better of a deal all the time. It's only intent that makes them wrong.
But intent is intrinsically hard to prove. If you want to nail a "poor black kid" for grand theft auto, you just have to prove that he has the car and it's someone else's car. The intent requirement (that he didn't do it involuntarily) is easy to prove from circumstances. If you want to nail a banker for insider trading, you not only have to prove that the trade physically happened (which is easy), but you have to prove that the banker knew insider information. And proving what was inside someone's head is hard.