Before the iPhone even came out it was obvious people would try to jailbreak them, and signed firmwares are a necessary part of jailbreak defense. You're right that Apple couldn't have been defending the App Store from jailbreaks before it was invented, but back then they did care about the carriers. They had a lot to prove. Now Apple is the darling of the industry and they have no reason to kowtow to the carriers any more, but they continue to fight jailbreaks to protect the App Store.