Why request the kitchen sink on startup? For example, if I have an app that posts on my wall when I perform certain actions inside the app, why not check and ask for that permission at that point? As a user the cause-effect will be clear since I've just performed an action that requires post.

The only benefit I see of the all-at-startup solution is that it provides one single "interruption" to the user. But the list of things an app might do can be large, even though it only may need to do some of them rarely. So if I see a big long scary list I'm more likely to just cancel out.