I don't think that Apple particularly cares about porting their software to Linux. Do you feel the same about Apple? That with such an attitude, they surely cannot succeed?
Apple releases a great deal of open source software, which, so far as I'm aware, all runs on Linux as well. At least Swift, clang, and LLVM, all run on Windows as well. So does their Objective C compiler, so of Apple's programming languages, that leaves AppleScript. I would not describe AppleScript as robustly successful.
I believe Apple could probably get away with keeping Swift proprietary, or only supporting Apple platforms. But they don't. I have no inside-track information on why that is, but I suspect the reason is fairly simple: developers wouldn't like it.
So the whole part of your message about "even the FSF saying that free software should run on proprietary system" works when you want to criticize Hare, but not when looking at Apple proprietary software, right?
A language is just another piece of software, I don't see why you should apply different rules to a programming language than, e.g. to a serializing system like Protobuf. And I don't think Google actively supports swift-protobuf (https://github.com/apple/swift-protobuf).
Hare upstream just says "we are not interested in supporting non-free OSes, but we won't prevent you from doing it". It's your choice to not use Hare because of this, but it's their choice to not support macOS.
> As a baseline, I support developers using whatever license they would like, and targeting whatever operating systems, indeed, writing whatever code they would like in the process.
> That doesn't make this specific policy a good idea.
