To lure US customers, app developers could offer lower prices to users who sideload, to offset the higher price paid for iPhones in the EU. App developers selling to EU iPhone owners will be able to avoid the 30% Apple tax and can pass on the savings to customers. Developers could offer more features to users who sideload. There are a variety of possible incentives developers could offer to lure users away from Apple App Store. That is assuming developers see the long term benefit of getting people to stop using the App Store and avoiding the Apple tax.
So app developers should bear the brunt of Apple's hardware price strategy? Seems unfair.
One of the complaints about Apple from app developers is the cost of doing business on the App Store (15% or 30% or whatever it is this week). So you're suggesting app developers charge less for side loaded apps: isn't that just accepting that the apple fees are fair?