It's not that simple. A broad match keyword, say "contacts" could still trigger an ad when someone searches for 1-800-contacts. Without factual Google data to back it up, how could one advertiser be 100% sure a competitor is bidding on their branded keywords?
Could one advertiser force a competitor to add a negative keyword for their brand?
What if I own a single word domain, which happens to be a very popular keyword? Could I stop people from bidding on it? I doubt it.
Google probably could allow advertisers to opt into some sort of trademark policy which would not allow others to bid on their brand, but there would still be limitations for many advertisers. Not to mention the the loss in revenue for Google.