This article is from the Macintosh system software FAQ, by Elliotte Harold email@example.com with numerous contributions by others.
Apple charges for system software because Apple's policy makers
suspect they'll make more money by charging for it than by not
charging for it. Apple is a publicly held corporation in a
capitalist economy where the law requires corporations to make
reasonable attempts to maximize profits. To give away something
Apple could make more money by charging for would be a breach of
the fiduciary responsibility of Apple's Board of Directors and
actionable by Apple stockholders in a court of law.