I'm curious as to why you think this is unique to capitalism. Politicians in every political system tend to help those who can return the favor. Whether it's communist leaders who award control of industries to their army buddies or socialist leaders who enact barriers to entry to advantage their schoolmates, this occurs everywhere.

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.