But if you support any sort of *nix, then you quite possibly work on OS X already. If not, there's still probably minimal work to do.
It's not like going from Unix to Windows, where you have an entirely different access control mechanism, system API, filesystem structure and path separator, etc.; it's just going from one Unix to another. So the author only has to accept a couple patches from his Mac-using colleagues, and then he's basically set for OS X. On the other hand, debugging a Unix app on Windows can become a full-time job, and may be more work than you can reasonably expect a few random patch contributors to accomplish.
It's not like going from Unix to Windows, where you have an entirely different access control mechanism, system API, filesystem structure and path separator, etc.; it's just going from one Unix to another. So the author only has to accept a couple patches from his Mac-using colleagues, and then he's basically set for OS X. On the other hand, debugging a Unix app on Windows can become a full-time job, and may be more work than you can reasonably expect a few random patch contributors to accomplish.