OK, but here's the obvious solution: These corporations are using tax-free dollars to run commercials for services the public obviously wants. By using commercials, the programs run the risk of altering their programming to "please the sponsor" (the whole idea behind public television is to avoid this; it's kind of like tenor in a university)
What if the government used the tax money they would be collecting from "tax-free" spots to fund PBS? My point is that, realistically, it's coming out of tax dollars anyway - just the back end. Write your congressman and tell him to get that money back and use it for PBS!
I absolutely don't want to lose these shows - but if we keep going down this path, how long is it before Cookie Monster will only eat "Chips Ahoy," and Oscar only use "Glad" trash bags? That's what companies like Sesame Workshop and others will face if there isn't more strings-free funding for public television.
PS. I think Nader's objection to the choice of sponsors is a little ridiculous, given the above obvious problem. Heck, my brief career was frequently funded by Phillip Morris.