As long as they clearly give the option, at the time of first setup - or an upgrade, to select which app store becomes default; and make it very easy to change default app store later, just like default browser, default search engine et cetera. But they must not be allowed to disallow for the reason that "you are on another app store, we don't like you, go away!".

Yup, it's then fair and they can keep the banner in their App Store that screams at font size 38 "This Journal App Is Da Best", "No Other Note App Has Been Made Greater Than This One".