It's hardly stealing if the original owners still have a copy. It's more of a breach of terms of service, or unlicensed access to intellectual property.
The "theft" spin on copyright violation is the result of years of lobbying by the media industry because they foresaw a drop in sales when people didn't need to buy the same thing over and over again.
Property is about who has the right to control something, not about scarcity. Although its original application was to a physical object, it's not limited to physical objects. So, "stealing" can be considered in both a narrow context and a broader context. The narrow meaning refers to literally taking a physical object away from someone else and giving it to yourself. But the broader meaning is wresting exclusive control of the property from the lawful owner. So, in that sense, copyright infringement and license violations can be legally thought of as theft.
Ignoring the legal and accounting details, the basic idea is that macOS is something that Apple includes with each Mac computer that they sell.
If you don't own a Mac, you don't own macOS. If you own a Mac and some other computer(s), you own macOS for the Mac, but not for the computer(s). You might've purchased an upgrade to a newer version of macOS, but if you don't already own a Mac, you don't have something to upgrade, so the upgrade doesn't grant you ownership of macOS.
Now, from a legal point of view, it's a good deal different from (and more complicated than) that, but that's the basic idea. So don't make the mistake of thinking that because Apple gives macOS away for free to Mac owners, and don't use elaborate and onerous copy protection or license-tracking nonsense, that you're entitled to install it on something other than a Mac.
I think that the illegal (ish) part comes from Apple making macOS free if you're installing it on Apple hardware. They don't publicly license it for use on non-Apple hardware, although, I would guess that VMWare if not others has a license to run it on random x64 servers to test ESXi with because setting up a different testing environment of Mac Minis to test every change to their hypervisor on instead of using their normal testing infrastructure would be dumb.
> I think that the illegal (ish) part comes from Apple making macOS free if you're installing it on Apple hardware.
I don't think it's true since the phrase in question was present also in Mac OS X when we had to purchase each version. Apart from that, this part of the license is not valid in several European countries. When you think of it, it's quite reasonable: how could anyone dictate how you are using something you purchased? It makes no any sense.
As with all information, society as a whole "owns" macOS - Apple only has been granted a temporary monopoly of this information in order to encourage them to create more.
Innumerable court cases have turned on what are often negligible practical differences. Apple is said, legally, to "hold" copyright. Saying they "own" it would mean they have all sorts of rights and powers around it that we as a society choose not to grant to them.
That is not to say that numerous judges have not, on their own initiative, elected to grant holders many such powers. Judges can be just as confused as anyone, and more than some, and so exceed their statutory authority. Congress, moreso. But there is still a difference.
Sure, and I didn’t argue for or against the use of that term. The legal definition of theft varies by jurisdiction, but is not necessarily limited to physical objects (e.g. services). I have no idea whether it’s appropriate in this case (I’m not a lawyer), but feels more like copyright infringement.
Control is a part of ownership. If you don't control how something is used and by whom, you don't really own it. So either using it in unauthorized ways is in a manner theft, or Apple's "ownership" is not real ownership. I'm not sure what the solution is, but IP is a tricky concept no matter your position.
It has nothing to do with actually preventing all infringement, but instead seeking enforcement in at least some cases. If a copyright owner is aware of substantial infringement and chooses not to pursue (or license) they may lose their copyright through genericization or other means.
(Caveat: I’m not a lawyer, but I learned broadly the above from an IP lawyer.)
The "theft" spin on copyright violation is the result of years of lobbying by the media industry because they foresaw a drop in sales when people didn't need to buy the same thing over and over again.