How is this in any way a secret? Sure, they don't go and shutdown sites and disrupt services for months while integrating acquired companies like Google does, but the bottom of the amazon.com home page lists their sites. Amazon buys sites that do well independently, and they do it publicly. For some anecdotal evidence, see the kindle for sale on woot.com when Amazon bought them.
"Secret" being a hyperbole, they meant "little known". I'm betting that if you surveyed the general public, few would be able to name even one of these other brands.
Arggh, language dilution is frustrating. I wonder if someday "secret" will colloquially come to mean "public information", similar to what happened to the word "literally". :P
