The Mac takes a lot of flak for being "the most proprietary computing platform" on the planet. In many ways, it is. But in the case of its source code, there's a heck of a lot of it available as open source, as this list from Apple shows.
Try to find a list like that for Microsoft's products, many of which (secretly) include open-source software. You won't.
Apple is about inventing new wheels, not reinventing old ones. Open source makes this possible. There's always a siren singing you to shipwreck...but this is no siren, and Leopard is no shipwreck.