Why is Corporate Money in Politics?

From Bloomberg:

After years of lopsided political giving to Republicans, American businesses are quickly rushing to support the new party in power. The top 25 corporate political action committees all gave more to Republicans than Democrats for the November 2006 elections. Afterward, 17 of them switched sides.

Why do we allow corporations to give money to political causes? We don't let them vote. If the company bosses want to give their own money, that's fine, but permitting direct contributions from corporations themselves provides for such a transparent buying of influence and favorable policies that it undermines the whole political system.