The issue is that donors don't have a _controlling interest_ in the organization.
Having said that, I would expect NPR to disclose, if editorializing a piece on Ms Kroc, the donation that Ms Kroc made to NPR (and they likely do that already).
They don't have control, but these foundations certainly have influence. Similarly for major advertisers, which also have influence in what is aired since editors don't want to anything that will alienate major sources of funding.
Having said that, I would expect NPR to disclose, if editorializing a piece on Ms Kroc, the donation that Ms Kroc made to NPR (and they likely do that already).