Amazon are correct, it is normal and has to do with the way the e-ink screen refreshes itself. Personally, I found that after I'd used my Kindle for a while I didn't notice it anymore, especially if I was immersed in what I was reading. I don't think it bothers most users.
For more information on how the e-ink screen works:-
http://www.kboards.com/index.php/topic,73465.msg1181739.html#msg1181739
For more information on how the e-ink screen works:-
http://www.kboards.com/index.php/topic,73465.msg1181739.html#msg1181739