I can grant an application permission to access my data on Facebook. I can't let an application get info about my friends though, that's their information and they control access through their own Facebook account. Otherwise, any single one of your friends authorizing an application would result in that application getting all your data. Obviously Facebook needs to err on the side of caution here, but hopefully there will be some new more restricted APIs in the future.

Oh, and as an added bonus they block all competition from accessing their incredibly valuable social graph, securing their monopoly for the future.