An author's copyrights can be assigned or transferred to a third party. This leaves the author with only the same rights as any member of the general public. (There are a few narrow exceptions, but nothing that would prevent the possibility of an author infringing on the copyright of a work he created)
It's also possible for a person who prepares a work to not be considered the author. This is the case for works made for hire.
And of course copyright isn't mandatory, though that just leads to works being in the public domain, so at least there's no danger of infringement there.
However, bear in mind that copyright only applies to original material, not to pre-existing material. A review which includes a quote is copyrightable, but the new copyright for the review only covers the portion original to the reviewer; the material quoted is only covered by the copyright of the work the quotes are drawn from.
17 USC 103(b):
The copyright in a compilation or derivative work extends only to the material contributed by the author of such work, as distinguished from the preexisting material employed in the work, and does not imply any exclusive right in the preexisting material. The copyright in such work is independent of, and does not affect or enlarge the scope, duration, ownership, or subsistence of, any copyright protection in the preexisting material.
Though there was a good reason for the original compact Macs to discourage users from opening them up -- there were exposed high voltage monitor electronics in there which could give you a hell of a zap of not properly discharged.
The later all in one Macs of the 90s were better in that regard. Their user suitable parts (motherboard, drives) all were easy to get at, but the monitors and power supplies were fully enclosed.
"Ninety percent of baseball is half mental." -- Yogi Berra