Because he is trying to write a C "primer" targeted at those who want to use Cocoa with Objective-C.
This is too simple for me, but I am an experienced C programmer. However, I've always disagreed with the philosophy that learning ObjC should require a thorough knowledge of C as a pre-requisite.
Teaching C with ObjC and Cocoa in mind will lead to a different kind of course with certain topics emphasized and others omitted or mentioned in passing. For example, a thorough treatment of the <string.h> functions would be a waste of time since NSString objects handle string manipulation differently. (I believe that to be an excellent programmer, one should be familiar with <string.h>, but that can be put off for a while)
Between you and me, I'd proceed a little differently, but that's another topic.