When you go to the supermarket...
When you go to the supermarket and ask where the Coke is the self is also full of other carbonated drinks placed on certain prominent points on the shelf by deals done between the supermarket and the manufacturers.
How is Ad-words really any different? And do the people who's branded web site was originally searched for think the Google et al should provide the service for free somehow?
No doubt the brands in question would throw their hochets out their landau if Google responded to this by removing every trace of their brand names from their search database.