I think they should have whatever ads they like. It keeps the site free for us to use and an advert is exactly that. An advertisement.
It isn't a royal command to go forth and purchase a big mac.
It isn't an endorsement.
It isn't approval.
It's an advert.
We are all adults with free will. Unless mcdonalds have some freaky mind controlling subliminal message embedded in their adverts, they have no power over us. 
I think people think MN is a multimillion pound operation that can afford to pick and choose advertisers with a lofty wave of the hand, sorry, you don't meet our criteria...
That's not the case. Someone once linked to their accounts.
They don't turn over a great deal!
advertisers pay the bills. I can't honestly say I have ever taken a blind bit of notice of any of the adverts on here though
I keep promising myself I'll contribute to the kitty by clicking a couple every day.
Of course, if people really hate companies advertising, we could always petition mnhq to let mumsnet become a subscription site with no advertisers...