I set up my author profile on Amazon because I wanted to associate my name with my books.
Now I can totally understand the conflict of interest inherent to an author commenting on their own work or that of others. But there is also the undeniable fact that authors are often the best champions of the works they like, that influence them, or represent the trends they admire. I've found many of my favorite books & authors by looking at what Robert E. Howard, Bruce Sterling, or Michael Moorcock have recommended.
I suppose in the long-run Amazon's rules will have little impact. The examples I cited came from posthumously published letters, fanzines, or book introduction. Still, I'd sort of like to be able to use that platform to tell people why Elmer Kelton or CL Sonnichsen or Barrington Bayley is awesome.