I like the McDonalds analogy, the only problem, when your local store kills someone with nuclear coffee, McDonalds national is also held responsible. If Continental could be sued and pay a heavy penalty for the Colgan accident, would they self regulate?
Brand recognition is only good as long as you protect the brand. If people begin to associate US Airways with cramped little commuters and a poor safety record, they'll choose Southwest.
This is the principle of a franchise. If the product is not consistent, it looses the benefit to franchise.