Apple's Xcode is a free download, the Swift compiler is open source. However, to become an "official" developer on the Apple platforms and publish apps you do need a paid developer account.
It makes sense for Apple though, because what the devs are really paying for is access to Apple's closed-source ecosystem, not the tooling itself. This company has no such market power; why would anyone want to pay for Zen, when it has no established ecosystem or audience?
And if you're doing ruby or nodejs or a handful of other languages, you can use the XCode CLI and most of your code (possibly all) will never see the App Store submission queue.
