They pay for political communication, whether mass communication or direct communication. Anyone in the business of changing minds and influencing behavior knows that this is expensive. The money to fund political communication must come from somewhere. Many reformers want it to be compelled, either by forcing broadcast outlets to give free air time to candidates, or by forcing taxpayers to fund campaigns. I prefer that it be voluntarily provided.
Total money spent is one thing, but how it's raised is quite another.
Raising money irks many politicians because it means they have to spend their time talking to their constituents and others whom their actions impact. In the past, soft money was limited to activities such as voter registration and get out the vote drives. McCain and Feingold want to prohibit political parties from accepting soft money and spending it on ads. This doesn't solve the underlying problem. It simply shifts it.