I have 3g right now and just tried to highlight something in the samples for "Song of Two Worlds" as well as "SEAL of Honor" and was unable to.
I also tried adding an annotation and when I started to type a message came up on the bottom of the screen that says, "Annotating is disabled for sample content."
Maybe you found that section particularly interesting and would like to look back at it later? Maybe you're reading the book for a class and instead of taking notes in the margin, you're taking notes on the Kindle.
I actually find myself highlighting stuff all the time. You can also see areas of the book that other people highlighted, which adds a kinda neat social aspect that isn't usually present in reading.
Did you mean to ask why is this a relevant comment? Because aaronsw mentions highlighting samples in the linked story.
Why would you not want to highlight something in a sample? There is no reason why text in the sample would be less useful. A quick example that comes to mind is the foreword/introduction of a book that references a book for higher level treatment of the subject matter or another book for that is better suited for newcomers to a field.