See here. Among Soros’s arguments:

The Bipartisan Campaign Reform Act was an attempt to limit the influence that special interests can gain by financing candidates and to level the playing field between the two parties. My contributions are made in that spirit.
President Bush has a huge fundraising advantage because he has figured out a clever way to raise money. He relies on donors he calls “Pioneers,” who collect $100,000 apiece in campaign contributions in increments that fall within the legal limit of $2,000 a person, and on those he calls “Rangers,” who collect at least $200,000.
Many of these Pioneers and Rangers are corporate officials who are well situated to raise funds from their business associates, bundle them together and pass them along with tracking numbers to ensure proper “credit.” They are buying the same level of access and influence for their corporate interests that they previously obtained with their own and corporate funds. With the help of Pioneers and Rangers, President Bush is on track to collect $200 million.
To counter the fundraising advantage obtained by this strategy, I have contributed to independent organizations that by law are forbidden to coordinate their activities with the political parties or candidates. That law minimizes or eliminates the ability to purchase influence in exchange for my contribution. Moreover, I don’t seek such influence. My contributions are made in what I believe to be the common interest. ACT is working to register voters, and MoveOn is getting more people engaged in the national debate over Bush’s policies.