Free Speech?

I was watching PBS the other day, on this occasion, I was watching the News Hour. In between programs, they had several "ads" I put ads in quotations because I'm not sure what the deal was because PBS is supposedly paid for by viewers thus giving them freedom to run whatever stories they want to without corporate interests dictating what stories they might run. I noticed a major trend in the companies that were sponsoring PBS. The ads ran in order: Intel > Monsanto > Chevron. Do you know what those three corporations have in common? All three are major I mean major human rights violators. I'm sure these corporations paid a fair amount of money to advertise with PBS, but thats part of the problem. I feel like if you're giving lots of money to a group or organization and have the power to say hey, if you run that negative piece about us, we're going to pull our funding. I cannot say for sure that this is the case but it saddens me that PBS could unfortunately be stuck to corporate censorship.

EDIT: Damn, after a little use of "the google" I have learned that Monsanto has sponsored PBS since 2005. Monsanto has also overseen the broadcast of a 30 minute program called "America's Heartland". It's in its fifth year of promoting monoculture farming, GE crops and the elimination of the family farm. I guess I'm behind the curb here.