By voting?
Voting is not the same as petitioning the government and that is protected in the US Consitution. It guarantees the right of people to petition the government individually or in groups to redress their grievances. When an American stands on the street corner within a sign that says: "Lower my Taxes", he is petitioning government. If he could afford it, he could hire a lobbyist who knows the ins and outs of Congress and goes there to speak directly with congressmen, senators, and the president to lower taxes. The constitution allows corporations to do the same thing because they represent groups of people like employees, employers, and stockholders.
Voting is not petitioning. After all, how else would government officials know what the people want? Additionally, in a democracy and representative government, people are allowed to complain to the government, march, petition, place ads, and make campaign contributions to support candidates who represent their views.