Well, perhaps they make verifying it hard.. but what is stopping you from publishing an app in the app store, while also hosting the source code for anyone to see, and use? 99 bucks a year?
You're already contributing a lot of personal investment to society with developing and maintaining FOSS and now you have to feed one of the world's richest companies' bottom line too just to make it available to their particular users? Yeah, no. Most FOSS devs simply won't publish there. Mac and homebrew can already be a pain, but iOS is just evil. Sucks for the users, but that's generally what you get for buying Apple. If they wanted this stuff on their platform, they'd make it easier, not harder. But they care more about making money with other people's software than offering a genuinely good service.
You're already contributing a lot of personal investment to society with developing and maintaining FOSS and now you have to feed one of the world's richest companies' bottom line too just to make it available to their particular users? Yeah, no. Most FOSS devs simply won't publish there. Mac and homebrew can already be a pain, but iOS is just evil. Sucks for the users, but that's generally what you get for buying Apple. If they wanted this stuff on their platform, they'd make it easier, not harder. But they care more about making money with other people's software than offering a genuinely good service.