That's a perfectly good justification, though: if you're feeling disgusted by people acting in their own interest, then you've designed the system of incentives (the thing that gives them interests to act in) wrong. To think otherwise is to be disgusted by the very concept of economics.
There used to be quite a lot of social value placed on people showing restraint. Having power, but not using it. Implicit in your argument is the assertion that you don't respect self-restraint. And when people stop respecting restraint, then people in power have no more reason to show restraint.
Reflexivity at it's best.
So, make that decision explicit. Would you prefer to live in a world where people with power restrained themselves? Or would you prefer to live in a world where people with power act with no more restraint than what is legal and in their interest? If the former then you need to stop using self-interest as justification for behavior.
Politicians openly game the system. They poll, then they change the message based on the poll, then they win. Then they do whatever they want since the platform on which they ran was a total fiction.
In truth, the only platform that really matters is the party platform. A career politician ignores that at his great peril, as they risk losing the support of an incredibly efficient, effective political machine.
And this is nothing new. The new thing is that, in an effort to feel sophisticated, more mainstream Americans are pushing aside the initial feelings of disgust. They tell themselves the story I've told above, and they tell themselves that that feeling of disgust is naive. It would be like being disgusted by gravity - totally pointless. The same thing occurs in the private sector, when bank leaders commit fraud and walk away. The fraud is explained away "well, he was just acting in his own interest" and the walking away is explained away "well, powerful people look out for each other."
The simple truth is that we need to stop pushing aside that feeling of disgust. It's there for a reason. We have an in-built sense of fairness that our government, and our business leaders, ignore at their great peril.