Technically speaking, it is not possible to give away, let alone sell, a working Mac without including the original or some suitable System/OS, and it is an offence to do so when the Mac was originally sold with one. However, were Apple so ill-advised as to seek to prosecute all of those who do so, and to take that course even further by prosecuting sellers of Macs that are 'loaded with yibbida, yibbida ...' in addition to an OS without transferring also the title to all the software, Apple would have no time for manufacture of new Macs. Unenforceable law is bad law.
That alone emphasizes the imbecility of the licensed software idea, quite apart from my long-held and oft-expressed belief that software licensing should never have been allowed to be established in preference to protection by patent (and royalty). We, the consumers, have only ourselves to blame in this regard, but who could have known what MS and others have since made of the idea?
And to forestall the riposte, having 50+ Macs of which more than 35 function with one or another of every System/OS from 6.0.8 to 10.4.x, each of which I have on multiple legitimate original media, I don't flout the convention. For example, I have six 'seats' each for 10.3 and 10.4. It is, nonetheless, only an established convention, not something that springs naturally—from the nature of—computing. And I maintain that the legal fiction of the 'reasonable man' supports provision of a System/OS on a working computer when ownership of the computer is transferred, whether that software is only installed, supported with a copy or an image, or supported with the original medium.
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