Presumably if MiccyD's advertises on MN the click will not be to a basic Ronald McDonald ad to buy happy meals, but an info site about nutrition, rainforest, packaging & litter etc...
I'm guessing that they are not trying to tell the gabby, opinionated, influential tribes of MNers about what McDs is doing on these issues.
Which is what conscious consumers want to use their consumer power for - to put pressure on McDonalds to do better.
Of course if you are a vegetarian, or completely opposed to capitalism and industrial food production you may think McDonalds is beyond the pale, can't be improved and should just be-bad-and-die.
Not sure that going back to McLibel is helpful here. The point then was that McDonalds reacted to criticism with paranoia and litigation. Now, if they are engaging with critics more openly through (for example) Q&A sites that they advertise on Mumsnet would be progress, no?