It is hard to argue with that reasoning. However, it is unfortunately not so simple as all that. If you accept campaign contributions and then vote against the organization (or company) that provided them, you can count on two things. No money in the next election cycle and if you get a contributor mad enough at you, you can count on money pumped into the election against you.
Rationalization sets in, even with people that think they are honest. They look at what they consider to be their mission, what they look to accomplish in office and convince themselves that for the greater good they have to accept the money and accept voting for something they know is wrong.
Multiply that a bunch of times over and you have our political system. It is corrupt and the tool of that corruption is the money. I agree with you that representatives should not compromise over concerns of future campaign contributions. But there are few in politics that actually perform that way.
You make a good case but I still am not swayed.
How would you propose that candidates finance their campaigns, if not by relying on funds from supporters?