This isn't necessarily a free software versus open source issue. Apple could have just as easily continued to ship a GCC version with OpenMP disabled. Sure you

This isn't necessarily a free software versus open source issue. Apple could have just as easily continued to ship a GCC version with OpenMP disabled. Sure you could build your own with-OpenMP GCC from source, just like you can build MacOS-compatible upstream Clang from source now. But third-party dev tools will always be a pain and an additional barrier to use. There are a couple of notable examples of major software shipping OpenMP-using code on MacOS, namely R and Matlab, but the structure of these projects means that few of their users have to encounter the complexity of getting an OpenMP-supporting toolchain working. Not so easy for a library author where nearly all of your users will need to use a special toolchain.