My experience at farmers markets tells me that I would be paying more than 100% more for the privilege of interacting with somebody that gets employed by the farmer instead of somebody employed by the store.

The analogy would work better if the grocery store sold me some sort of platform for cooking food, that only worked with certain types of food... I don't know, it's hard to find the analogy compelling at all.

People develop for the App Store because they get access to a particular customer base. I could definitely support some sort of customer protection that forces side loading options (but they must be options and easy to disable and hard to enable, otherwise it destroys one of the best aspects of the phone.) But as far as caring for slight changes in percentages for app developers, well, I don't care about that one bit at all, anymore than I care about Apple getting a "fair" deal from the other businesses it interacts with.