Have you heard of a little something called Fair Use? If you were to modify one of Apple's system utilities and then distribute it, you could easily run afoul of copyright. Simply making a local modification in-place, however, is no more illegal than installing a new hardware driver, changing your shell prompt, or downloading and using Firefox in place of Safari.

Now, if you changed some piece of the system in order to circumvent FairPlay, or some other DRM component, then you might be running afoul of the DMCA. Not the same thing as copyright violation, however.