Call of Duty 4 is written for windows and DirectX. It's then wrapped inside Transgaming's Cider virtualization engine, which maps directx and win32 calls to OpenGL and OS X, introducing CPU overhead and often loss of fidelity (e.g. pixel shaders).
Good to see Apple pawning cheap windows ports on their app store. And from its success, it sounds like consumers are eagerly shelling out $$ for degraded gameplay.
I don't know where you got your information, but it's wrong.
Call of Duty 4 is an Aspyr port, that means this is a 100% native Mac port as Aspyr ports everything from scratch. Aspyr isn't using Cider crap in any of their games.