Simplistically stated, perhaps, General motors isn't selling cars at Test Track (or the World of Motion), while McDonald's is offering standard menu items for sale. They are not just sponsoring Conestoga Fries, for instance - Conestoga is selling McDonald's products under the McDonald's name. You couldn't find Kraft or Nestle products for sale in The Land, no General Electric light bulbs sold in Horizons, and no Exxon mini-mart (or gas) available over at Universe of Energy. Those are sponsors, 'selling' the corporate name or image maybe, and certainly benefiting from good publicity and promotion, but they don't have a store set up in the park.
McDonald's does have stores set up in the parks, even if it is only a fry cart. That crosses a line, from sponsor of a Disney establishment, to having an actual retail presence in the parks. I might not have a problem with McDonald's sponsoring a Disney counter-service restaurant, but it couldn't be a 'McDonald's' restaurant offering a McDonald's menu. For instance, if the only change made to Flame Tree BBQ was to add "sponsored by McDonald's" to the bottom of the sign - and the menu remained unchanged - that would probably constitute a sponsor arrangement, and might be completely appropriate.