They can all be bought. Again, they have A VESTED INTEREST in doing what the lobbyists want. It doesn't make them "corrupt." It makes them human.
Pretend that you work at McDonald's as a manager. Suppose a customer comes in an puts in an order for 100 hamburgers, and he wants tomatoes added. Normally this is $0.50 per burger, but he offers you $40 if you'll let him slide. Would you do it? As a manager, you have the power to make that call, but you're still costing your employer more money by allowing it. Does that make you corrupt?
Likewise, a politician has the authority to vote based on whichever way he chooses, and if he is being paid for his vote, then so be it. That doesn't change the fact that he is still acting in accordance to the system. He might cost us taxpayers more money, but I still wouldn't consider him corrupt. The lot of us would make the same decision if put in the same situation.