Check out this post on corporate philanthropy. However, here's another factor to consider: Think of all the money that corporate America donates, and think of the recipients of that largess. Think of groups on the left, including universities and institutions like NPR and PBS, who happily solicit and rely on corporate money to keep them in business. Yet they use this money to regularly rail against corporations.
Can you imagine organized labor giving one dime to a group with which it disagreed? They are notoriously heavy handed, enforcing dogmatic purity on any group that receives their lucre. Kinda like the Mafia. And while you're at it, imagine the outcry if corporate America decided not to fund groups that used their money to undermine their good work and reputation. You can hear the howls of censorship now.
Just another case of the great double-standard. Let the unions weigh in and regale us with all the groups they fund that criticize them -- or how 'bout one that even disagrees with them slightly? Don't hold your breath.