That's what we call capitalism. The difference is that politicians are supposed to represent the people, not just the people with money, but in practice they are also capitalists.
At least when it's pay-for-favors the public can follow the money. Not to mention the fact that under this system, any ordinary citizen has a much better chance of affecting outcomes by encouraging others to pool resources than he would have under systems that afford people power mainly, or solely, through kinship or affiliation.